Araragi's having a hard time as a young cop. He officially passed the national exam and everything, but is looked at as a sarcastic/disagreeable elite (perhaps due to his parents?) so his work environment isn't pleasant.
Suou Zenka is also a policewoman. 10 years ago she was a swimmer with a promising future in the sport, but was grievously injured after being hit by truck-kun who had run a light and then swerved to avoid hitting some elementary school kids. Eating mermaid flesh saved her and made her immortal, but in exchange when she gets wet her body turns into a fish, so she had to give up on her dreams. Not like the typical "little mermaid" mermaid, with a pretty fish tail, but full on mostly fish creature like a sahagin or something.
This makes life extremely difficult for her, since she essentially has to avoid water. Washing her hands turn them into fins, she can't ever go outside when it's raining, etc. The amount, depth, temperature, composition of the water determines what sort of fish she turns into. She has even turned into hyper-creepy-alien deep sea fish before.
Furthermore, now that HER body is all "mermaid meat", it makes her flesh extremely desirable to other kaii/oddities.
Summary: 5 kids have drowned in the river by Naoetsu High School recently. The serial incident started with one boy being attacked and surviving, but he's still severely injured and unconscious. 3 witnesses to the unconscious boy's attack testify it was like he was 'dragged into the water by invisible hands'.
Koyomi goes swimming in the river without incident, and so turns to Shinobu for advice, who tells him that the attacks will indeed continue until he deals with the oddity causing them. With Suou using the mermaid flesh of her own body as 'bait', they lure it out. The true form of the "culprit" is the soul of the very first victim, the unconscious boy. When he drowned naturally, his soul was assimilated into the river itself and has unintentionally been drowning people while desperately seeking help.
The reason the boy's soul didn't react to adults and only "attacked" kids was because his parents had badly neglected him and so he didn't trust adults. They're able to draw him out now because they visited his body at the hospital, I believe.
During the climax, the oddity's power prevents RRRG from diving into the river to help. So he has Suou help out by using one of her innate mermaid ability, the "Mermaid's Song" which affects "any and all Humans who are drowning". Thereby Koyomi's able to grab onto the boy's soul's 'hand' and pull him to safety, resolving the incident.
Notes of Girls Appearing in the Story:
Karen: Tsukihi is away overseas and their parents were transferred downtown. She now lives alone with her beloved Onii-chan. She's also a police officer stationed in Naoetsu as part of Public Safety, which occupies her days along with her thriving martial arts. She sucks at housework, but her cooking skills have risen significantly due to her taking care of Koyomin's meals when he was in training. She's still growing taller and markedly over 180cm now.
Kanbaru: Araragi meets her when he goes to the hospital to pay a visit to the unconscious drowned boy. Her hair is all the way down to her waist now. Due to her own experiences and what happened in Rouka's past, she's aiming to be a sports doctor. She's playing basketball on her day's off with her colleagues, but entering university taught her how narrow her outlook really was. She still meets/talks with Ougi from time to time.
Gaen: Doesn't appear personally, but does in reminiscences. Her idea is to take "the practices of the specialists behind the scenes and create a public organization to do the same work." So she collected/scouted people who had experiences with oddities in one place and set up shop.
Shinobu: The same as always, apearance and personality. The one difference is that in the past she always wore that sheer one-piece dress, but now she alters her 'costume' to mirror whatever Koyomi's wearing. If he's in a winter coat, so is she, if he's in a swimsuit, so is she, etc. RRRG regrets how things turned out when he over-utilized her powers so much in the past, so he's taken a hard-line stance that he can have her help out with advice and whatnot, but not cooperating like in the past.
Kizashima Nozomi, 29 years old. Police inspector, and golem. She looks like a petite, baby-faced woman, but in truth she died of an illness when she was an elementary schooler. However, her grandparents delved into forbidden arts and affixed her soul to a clay doll, reviving her.
In the years after, her grandparents would adjust and "age" the clay body as needed to suit what her proper appearance should be, but then they died when she was in middle school. Since she presumably doesn't know the magic, she hasn't been able to alter her body since then and so remains in that appearance for good.
She has no idea how long her soul can stay affixed to her clay body, but in essence she's immortal. Likewise, she has no sense of pain whatsoever. However, since she IS made of clay/mud, she will begin to crack when dry, so she always carries water with her (kind of the opposite counterpart to Zenka). Her personality/speech patterns resemble Mayoi a bit.
Summary: There's a "Serial Slasher Incident" going on as students coming home from Naoetsu high are having their backs slashed up, although only their uniforms (no bodily harm). None of the victims have any idea when they were even slashed. Ararararagi and Nozomi go to investigate his alma mater together in case it's a kamaitachi (sickle weasel) or the like.
Araragi consults with Ougi, who still resides within the non-existant class 1-3 at the school, and comes to the realization that the majority of students being slashed are on the way HOME from school, not those coming TO school, so he and Nozomi take to watching over the students at that time.
Koyomi's perched across from the convex road mirror. Nozomi can dowse herself with water and turn completely into a flow of living mud, which can assimilate with the earth around her for camouflage. But nothing happens. From Ougi's hints, Araragi guesses that the students on their way home from school only coincidentally REALIZED that their uniform's backs had been damaged because they saw them in the reflection from the street corner safety mirrors. The actual 'scene of the crime' was AT school, when the uniforms were changed for gym, which is why "only the clothes got slashed, and not the people in them". So just a Human crime.
Notes of Girls Appearing in the Story:
Spooky Ougi: Has become one of the 7 mysteries of the school, along with the classroom. Seems like she leads people who are going astray further astray. Her outer appearance hasn't changed, but her personality on the inside has dramatically changed, according to RRRG, she's ended up completely unmanageable/uncontrollable/spoiled. However, he surmises that she's the one who actually ended the case within the school so that the damages to uniforms stopped occuring.
Has already left the city as of the events of Musubi. Has helped Gaen out as detailed in NadeMonogatari, and has debuted as a manga-ka under the pen name Sengoku Nadeshiko. She's got three serialized works, although Ougi says their not very popular. However, she adds that, though they're cute they've got an underlying darkness in them that gives them cult popularity status.
Saizaki Mitome: 29 years old, police inspector, descendant of werewolves.
Though she comes from a long bloodline of lycanthropes, she differs in that she was not born inherently an oddity/monster. Additionally the way of the wolf is strong with her, and she can transform into wolf form not just from seeing the full moon, but by looking at ANY round object. Conversely, even under the most powerful of moons she can resist the transformation. While she has quite high physical skills, that's by Human standards and she's not altogether inhumanly strong or fast.
While she's got a rough personality and gruff manner of speeking, she carries an absolute conviction in completing any mission she's given. She was actually originally hoping to be a police dog when she aimed for the police force. It's for her mental strength, not her physical strength, that Gaen values her. By the age of 23 she was already commanding her own troops in emergency disaster relief and large-scale criminal investigations.
Hanekawa Tsubasa (written in katakana, unlike the usual kanji:
The same woman we knew, but it's more appropriate to call her "a true bakemono" these days, hence the katakana. As detailed in WazaMonogatari, she travelled the world with Dramaturgy, and it wouldn't be wrong to literally say "she toured around every and any country in the world, and travelled to them all".
After conducting various peace-keeping activities such as landmine removal, she has become an international celebrity and is reknowned as the "japanese Jeanne D'Arc". Since the age of 20 she goes around, not advocating or acting on behalf of the victims of war, but as a 'war mediator' in the interest of halting War itself. She has brought countless wars to an end around the world as she brokers peace treaties.
However, as an extreme pacifist who is as close as possible to believing in and pushing her idealogy of "erasing all the borders between countries of the world", she already is seen as walking the edge of the knifeblade of desiring world conquest herself. She is therefore treated now as an international wanted criminal offender by every nation in the world. ::A lot of people seem to comment that Hanekawa seems the most changed by the 5 years, and that her premise is crazy.::
Summary: Hanakawa tsubasa has become an internationally wanted person of interest, and so she has returned to her home country of Japan in order to completely eliminate all traces of her past presence. Mitome, who's been appointed in charge of Hanekawa's guard, goes to talk to Ararararagi and ask him about Tsubasa as he knows her.
Mitome asks him,"During her stay, do you think Hanekawa will come see you?" to which Araragi responds,"You don't need to worry about that." He returns home to find that Tsukihi has come back from overseas. He thinks he's going to settle down to a nice meal with both his sisters for the first time in a while, only to find that Hanekawa is already there and staying over.
After dinner, the two of them are washing dishes together when Hanekawa tells him,"I came back here just to see you, Araragi." "Why would you do a thing like that," asks RRRG, as well as,"How did you escape your heavily guarded hotel?" "If you can see through that trick," she replies,"Then I'll answer your other question." After the short conversation, and after leaving the Araragi house, he reports to Mitome.
Araragi concludes that the trick she used was sending an exact body double she'd found whilst traveling the world. Furthermore, he concludes, the real reason 'Hanekawa' came to the Araragi household was to reclaim, from his room, the panties he'd gotten from her in the past "in order to erase all traces of her past existence" as she'd intended. However, Mitome responds with,"The possibility exists that I could have been guarding the body double, while the REAL one might have been the one who came to see you."
Despite pointing that out, Koyomi insists there's absolutely no way THAT'S true. Because, as their positions in the world stand now, he's just an inconsequential man to her. But whether that makes him happy or not...
Notes of Girls Appearing in the Story:
Dropped out of college twice, now studying dance overseas. Her current plan is to end up as a casino dancer. Her current hairstyle is just like original Hanekawa, twin-tail thick braids. Seems like she's still in touch with Nadeko.
Kouga Tsuzura: Section Chief of the Hearsay Department. Only pure Human among the members there. Valued by Gaen for her great skill at communicating with people, and because she wanted someone with absolutely no spirit senses to command the department so as never to be influenced by oddities.
She values Araragi's abilities, and has told him she wishes him to take over as chief of the Hearsay department some day. However, she doesn't believe he necesarily lives his life in an ideal manner, and has told him to could live a bit more relaxed/comfortably if he wanted.
Senjougahara Hitagi: Honestly, most of the relavent info here is already known. The first two times she and RRRG broke up and got back together were in college. The first time it happened was due to "extreme complications" when Araragi reunited with Sodachi in university and let her live with him at his parents' place, but she eventually let it slide and made up with him after making a show of threatening/intimidating Euler.
She is qualified as an accountant while studying abroad, and currently works as a local civil servant at Naoetsu City Hall. She chose that location because she was indebted to the woman in charge of her social welfare there when she was a student. She took out a civil servant loan and purchased the same abandoned house where she and Ararararagi met as kids to study math, and now lives there. She is the picture of a civil servant, her hair tightly done up and she has become a meganekko (glasses, ho!).
After graduating from high school, at Hanekawa's prompting, she took the entrance exam to a certain university's math program, and there, surprise surprise, she reunited with RRRG (fucking Hanekawa). She had trouble finding a dorm/boarding house to live in, and when Koyomi consulted his parents about it they basically ordered him to let her live at home with him (paralleling her living there when she was a kid). This did NOT go over well with Gahara-san, since RRRG hid the fact for a bit, so she broke up with him and brought her wrath upon them. In the end How-Much threatened to jump off a building and commit suicide in order to get the two of them to talk and try to make up. Oikura immediately left the house and then helped the two get back together
For a while the three of them enjoyed a normal college lifestyle together, but after Araragi broke up with Senojougahara for the second time over stupid, worthless reasons she was so angry at him that she 'permanently' ragequit any relations with him forever for the 4th time. It was October 13th. Even after reuniting with RRRG her sharp tongue and pretending to hate him didn't change, and she later once shared her inner thoughts by commenting,"Go chase after Hitagi overseas, and afterward suffer some catastrophe and be lost by the roadside somewhere!" Now and then though, she'd give advice about the breakups, and afterwards she exchanged phone numbers with him again and rescinded 'cutting off their 'friendship' forever'.
Although RRRG and Hitagi had broken up twice before, when she began officially working as a manager at her finance firm it caused strife and they had a huge fight over the phone. By the time it was over, they'd broken up for the 3rd time. Sometime afterwards, he was submitting some documents at town hall when he ran into Sodachi. As he was attacked by her barbed tongue like usual, she eventually gave him some advice about dating Gahara-san and their future together as he wondered what on earth he actually wanted to do.
Koyomi decides to head for the North White Snake Shrine, which he admits he's been avoiding for awhile because I believe he feared his adult self wouldn't be able to see Mayoi anymore. He resolves,"I am going to hold my wedding ceremony to Hitagi there".
Seeking Shinobu's cooperation, he asks her to prepare a survey report on oddity activity in the city center, and then goes to ask Section Chief Kouga to recommend him for overseas training. Additionally, he presents that,"Since Shinobu is with me, we can act as interpreters for foreign oddities". Anything and everything to be with Hitagi.
Upon receiving his endorsement and returning home, he finds Senjougarahara there waiting for him. The final chapter/couple of pages have already been translated and are heartwarming, but essentially, while he was planning on ditching Japan for overseas, Hitagi got her company to open a Japanese branch so that she could move home and live together with him again. They were thinking the same thing and working on it from both sides. They make up and have a reversal of the scene at the end of Mayoi Snail where RRRG goes <I rab you> in engrish, and Hitagi goes "Koyomi fascination/tore". The indication is that he will ask her to marry him soon.