3 and 9

V1-3:Covers 3 and 9's assassination of Haldur.
V4:Covers 3's backstory of how he got into the assassin organization and his failed escape attempt with Ace,forcing him to kill the latter to survive after Ace seemingly tries to kill him to get spared as the rules of the organization allows one to be spared from execution if one kills their partner in crime.
V5:Covers 3's attempt to escape to Liberl but got caught by Emperor and Nine because the pseudonym he used for his ticket was one he used in one of his mission with 9,ends with cliffhanger of Swin stuck in a pinch.
V6:Covers 9's backstory,ends with 9 backstabbing Emperor to save 3.
v7-9:Covers 3 and 9 vs Emperor.
V10:Covers the ending of the story.
Three of Swords:
Real name is Swin.
13 years old(in the book,the real Swin's current age is unknown).
1 year older than Nadia.
Has been an assassin for 3 years.
Has hair bangs long enough that you can't see his eyes unless he sweeps them to the sides of his head,the real Swin's mekakure isn't as long as the book version or he had a haircut by the time Hajimari no Kiseki begins.
Good at using a sword,bad at using arts.
Wields a longsword and a shortsword which are actually 2 parts of 1 weapon that can simulate the effect of an explosive Art by cutting the same place with the 2 weapons and cutting it again with the combined weapons(Black Workshop S Weapon?),what's dangerous about this simulated effect is that it can cause the explosion from inside the target so even armor plating can't defend against it.
Couldn't take the pressure of continuously killing people and wanted to escape the organization to become "human" again.
Never knew his own mother,nothing was mentioned about his father.
Can control his emotions during battle,a skill only a small amount of high-level martial artists only achieved.
Not really friendly towards other people because of the Ace incident.
Bad at putting up an act.
Unlike Nadia,Ace and The Emperor,Swin's life before getting recruited by the organization is a total blank(not even his backstory in V4 gives even a hint about it,it's like the author of the books doesn't want to address Swin's life before he got recruited by the organization),though considering he never even once thought of going home in the books,it's doubtful there's anything remaining of his old life
Joined the organization when he was 7,graduated from the training school and was given the name Three of Swords when he was 10 and partners with Nadia for a year(how much time he spent with Ace as his partner is never addressed)
The act of killing Ace caused him bear a darkness in his heart(Enforcer flag)
Nine of Swords:
Real name is Nadia
12 years old(in the book,the real Nadia's current age is unknown)
Often refers herself in third person.
Calls Swin Su-chan and herself Na-chan.
Doesn't consider her father as family because of the abuse she suffered under him and as far as she is concerned,Ace and her mother is the only family she ever had.
Holds a teddy bear with her at all times which is actually hiding an Orbment(and somehow a rubber boat even though it's normally impossible to fit one inside a teddy bear,so the insides of the teddy bear is some kind of pocket dimension to store items?) inside it and also uses a knife and steel strings as her weapons.
Good at using Arts.
Face looks like a doll and has sapphire colored eyes.
Suffers from narcolepsy sometimes but she's actually just half-asleep in which she analyzes the enemy and pinpoints their weaknesses
Highly talented genius,capable of precise analyzing in moments and graduated from the organization's assassin school in a year while normally it takes 3-5 years for one candidate to graduate.
Has parents and a brother who is Ace,mother died during childbirth and Ace sacrificed himself to make sure 3 is spared from being killed by Emperor,her abusive father sold Ace and later 9 to the assassin organization through a broker.
Became 3's partner to keep an eye on and save him as he is the person her brother sacrificed himself for.
The Emperor:
Caretaker of 3,9 and the deceased Ace in the books.
Wears a cloak most of the time(to the point Swin commented Hermit would be a more fitting name for him rather than Emperor) which hides his Artifact to make sure the Septian Church doesn't get on his case.
Supposedly implied to be a prince of a kingdom before the citizens went radical and overthrew him from the throne because they're taking their frustations of his deceased father's mismanagement on him.
Wields an Artifact known as the Monarch's Regalia(consisting of armor,mask,orb,and scepter) which allows him to absorb and manipulate gravity.
Notes and speculation:
The events written in the book are stated in an interview to be mostly historically accurate but exactly what is the fictitious element in the book is unknown.
The organization in the book is a Calvardian organization connected to Ouroboros but not a subordinate organization,this doesn't rule out the organization being a collborator with Ouroboros however.
Exactly when the first 3 and 9 book was published is unknown but by 1206,the book series is critically acclaimed by the public and known in all of Zemuria.
The Emperor is very likely to be the fictional element in the books as the kingdom he was kicked out from is heavily implicated to be Calvard,and the book treats as if The Emperor is allegedly the same prince that got overthrown a century ago which is impossible unless The Emperor is either an archaism made in the image of the prince or a clone of the prince implanted with the memories of the original?(not impossible considering Ys 9 had homunculus duplicates of Adol,Marius,Chatelard,etc created using parts of their original's souls and Falcom can use that plot point again in Kiseki)
Swin's situation in the book fits the meaning of the reversed Emperor arcana(powerlessness,being untrusting of others,hating the rules of the organization,etc) and while The Emperor in the 3 and 9 book represents the domineering aspect of the reversed Emperor Arcana with him being controlling of the assassins in his command instead of doing what is best for them(i.e letting them go),he didn't abuse his power since he follows the rules of the organization to kill any deserters,the reversed Emperor can also mean being powerless in the system you are in and rebelling against it instead of just an higher authority doing power abuse and depending on the circumstances the Reverse Emperor requires you to stand up to your higher-up to get a more flexible environment and assert your own power which is what Swin does to get out of the environment of continuously killing people and become a "human" again.
Nadia is a mix of the Upright and Reversed Moon Arcana,what with her struggling to tell the truth about Ace to Three and deceiving Emperor and Swin in V5 and 6 under the illusion of her ratting Swin out when she was actually trying to get Swin to safety(Upright) and giving clarity to Swin regarding the truth about Ace's supposed betrayal and releasing him from his guilt regarding Ace's death in V7(Reversed).
The real Swin and Nadia accepted a job from someone in Hajimari no Kiseki to transport a suitcase until they were presumably attacked by Zoa Balor until C saves them by sheer coincidence and Lapis "wakes up" as a result,which led to them being hired by C to become members of the 2nd ILF.
In the Sinner's key visual,Lapis,Swin,Nadia and Rean are shown to be generating noise while the SSS and C are the only ones who aren't generating noise,Rean(and by extension Crow and Millium) likely has to do with the fact that normal ending Rean is the original Rean while True End Rean is more or less a duplicate that came about because the Grandmaster rewrote the timeline so that the normal ending and everything that happened after never happened,Lapis is because she isn't human but an entity presumably from outside Zemuria using a modified Rozenberg doll as a vessel,so the one thing both Lapis and True End Rean has in common is that they are existences unnatural to Zemuria so Swin and Nadia's existence are also unnatural but why exactly is unknown,could this be connected to the fact that the 3 and 9 novels never gave descriptive exposition regarding their physical appearences even though the books are actually their wanted posters?