Kingdom Hearts 3 spoilers

True Organization XIII's roster in KH3:
-Master Xehanort
-Young Xehanort
-BBS Terra-Xehanort(called the final member of the True Org because he's the last one to time travel to the present)
-Seeker of Darkness
-Xemnas
-BBS era Vanitas
-Xigbar
-Vexen(Double agent trying to complete the Replica program and backstabbed Xehanort later on to save Ansem,goes back to being Even again in the ending)
-Saix
-Demyx(benched in favor of Repliku,defected because Vexen bribed him with a job that doesn't involve getting yelled at or fighting)
-Luxord
-Marluxia
-Larxene
-The version of Xion back when she was just inducted to the Organization.
-CoM era Repliku.
Anti Aqua is Aqua after being corrupted by SoD to protect Ansem.
Vanitas followed Aqua and SDG to the Land of Departure where he attempted to absorb Ventus again only boxed in and beaten up by Aqua until he plays dirty by trying to kill Ventus's body knowing Aqua would take the bullet for her friend at which point Ventus beckons Sora to use the power of Waking(which Sora never actually lost but just locked out from using until there is someone the power can be used on is near) to awaken him and save Aqua.
-The time reset that happened in KH3 is because Sora used the Power of Waking to put his friends's hearts back where they used to be which caused a singularity that resulted in the bad ending written in the BoP to have never happened in reality.
-Demyx,Luxord,Marluxia and Larxene returned into becoming humans again after their first defeats and they became Nobodies again to join the True Org XIII,by the end of the game all 4 of their Nobodies have perished as confirmed by the ultimania but it's implied they're not restored yet.
-The Power of Waking is the power to use people as portals to access their Station of Awakenings to wake them up from their sleep,in other words it's a method of traversing Worldlines of Sleep.normally this power has no drawbacks,but if one uses this to use the Stations of the Awakening to locate a person risks falling to the abyss of sleep or getting removed from the time axis.
-The Replikunort and Youngnort subplots in BH6 and Toybox was part of a plan to perfect the Xion duplicate Replica to replace Sora,it backfired when Sora said her name in the climax which was the spark needed for Xion's heart to inhabit the replica which ended up sending past Xion's heart back to the past and reviving the present Xion.
-A man wearing black clothes divulged the existence of the black box to Maleficent(who still has memories of time travelling in her unconsciousness) to set her on the trail of the box so she could obtain it,Luxu caught them in the act of trying to find it in Olympus and responded by using Luxord as his proxy to kill them before they found out the box is in the Graveyard.
-Isa returned to being Saix to atone for his sins by inducting Vexen to the true Org so that he can create vessels for Roxas,Xion and Namine,he was revived in the ending after he dies as Saix again,eating ice cream with Lea,Roxas,Xion,Hayner,Olette and Pence in Twilight Town in the ending.
-Scala ad Caelum is the nexus and source of all the worlds that came after the Age of Fairytales(not because it literally created all of the other worlds but because it was ground zero of the Keyblade War which caused the world to be shattered and made the broken off pieces into independent,closed-off worlds) and the (upside down) ruins of Daybreak Town's clocktower in the final battle reveals that it is the rebuilt Daybreak Town after the post-AoF world reorganization.
-The Chirithy that shows up in KH3 belongs to Ventus and goes on to live with TAV in the Land of Departure in the ending.
-The time travelling members of the True Org(Xemnas,Youngnort,SoD,Vanitas,Xion,Repliku and Terranort) were able to stay in the present by either being anchored using hollow Replicas as vessels or using Present Terra's body as a vessel(Terranort),Present Repliku exorcised his past self's vessel so that it can be used to revive Namine in the ending.
Axel and Saix's dropped usurpation subplot back in Days was for the purpose of finding a missing girl who has the memories of the Keyblade War(heavily implied to be Skuld),but because of Saix being jealous of Roxas and Xion and the misunderstanding that he and the girl was supposedly abandoned by Axel and replaced with Roxas and Xion,he willingly became a member of the True Org in a last desperate attempt to find the girl.
-Demyx,Luxord,Marluxia and Larxene were recruited because of them having a connection to the ancient Keyblade wielders.
-Roxas was revived by using a Replica sent by Ansem and Ienzo to the Graveyard and the connection he has with his friends to bind him to the Replica vessel.
-Xehanort(who was convinced to accept his defeat and let Kingdom Hearts get relocked) and Eraqus(whose heart was within Terra) passes on in the ending.
-Book of Prophecies only recorded up to the moment before the history negation.
-The ritual to properly forge the X-Blade involves the 7 lights and 13 Darknesses to clash 13 times to create replicas of No Name to forge the X-Blade,the conflict doesn't need a winner and loser as MX created the final dupe without Sora,Riku and Mickey losing and dying.
-Kairi seemingly dies in the climax but it turns out Xehanort just shattered her body and she came back in the ending after Sora reversed the damage using the Power of Waking.
-Vanitas went to Monstropolis to absorb all the leftover scream energy left to fill up his incomplete heart(as he is Ventus's darkness,he's missing Ven's light to make heart whole) and wants to absorb Ventus's heart to become a complete person.
-Ephemera shows up as a vision in the climax,helping Sora and co. by animating the Keyblades from KHUX Union members.
-Marluxia completely forgot about his original identity as Lauriam,his sister Strelitzia and his comrades but still retained his knowledge of the Keyblade,when he finally dies in KH3 he remembers everything and thanks Sora for helping him remember.Ultimania is vague on whether Larxene(Elrena) remembers but it said she joined Org XIII because she drawn to something in it.
-The Guardian's true identity is that of a prison to enslave Terra's heart,he took control of it to save Ven and Aqua from dying and to exorcise BBS Terranort from his body and back into the past.
-The orchestra conversation between Lingering Will and Namine came into play when she contacts it to bail out the Guardians from being maimed by Terranort,it disappears off-screen when Terra gets his body back but he can still summon and use it.
KH3 Secret Report spoilers:
Luxu is still alive in the present day by bodyjacking countless people to fulfill the final mission entrusted to him:to awaken the Foretellers from their sleep,his current identity is that of Braig and the reason why he is putting up with Xehanort is to observe him as he fulfills his destiny and to reclaim No Name from him.
No Name was passed on to a Union Leader(unspecified,but current evidence suggests Blaine was the one who inherited the keyblade) to create the cycle of bequeathing No Name until it ends with Xehanort.
Luxu was aware that an impostor had replaced Strelitzia and that this faker planned to bring the Union Leaders to another worldline to escape the destruction of the data world,as even though Dandelions escaped to a data recreation of Daybreak Town to escape the apocalypse brought by the Keyblade War,darkness will inevitably descend upon that world to destroy it.
It was recorded in the Book of Prophecies that No Name will continue to be passed on until a chosen one of the Keyblade aligned with darkness will inherit the Keyblade and reenact the Keyblade War to unlock Kingdom Hearts as part of Sugita's program,this chosen one was Xehanort.
4 out of 5 of the Union Leaders were all blown away to separate time periods to escape the destruction of Data Daybreak Town with KH1 era Maleficent lending a hand to this.
The missing girl referenced in KH3's story is all but confirmed to be Skuld.
Lea and Isa's attempted break-ins to Ansem's castle was to find out the truth about the human experiments inside the castle,on one break-in they met Skuld and eventually became friends with her,Lea wanted to save her from imprisonment but by the time they went inside again,Luxu had already taken her away to somewhere unknown.
Eraqus is descended from the first Keyblade Master from the Dandelions(HEAVILY implicated to be Blaine) that goes on to be Scala ad Caelum's founder.
KH3 Vexen is Even becoming Vexen again to complete the Replica program to atone for his sins.
The only memories the missing girl that was designated "X" before awakening in imprisonment were 4 other friends and a key.
Before their restoration in the story,Sora harbored Ventus,Roxas and Xion's hearts within him but by KH3 they had been with Sora for so long that their hearts have melded with his and have gone silent,what is needed to revive them are vessels that can house their hearts and a spark that wake them from their sleep,as Ventus's body is still intact in Castle Oblivion all that is needed to do is to create vessels for Roxas,Namine and Xion by using records from the data Twilight Town to reconstruct their bodies using the perfected Replica shells as molds.
Before Xion,Vexen originally created 20 replicas with Repliku being the only success,Xehanort used the failed batch(except Repliku who died in CoM) and post-Xion Replicas as vessels to anchor Xemnas,SoD,Youngnort,Vanitas,Xion,and past Repliku in the present with the help of Vexen who secretly used this opportunity to perfect the Replicas to atone.
Aside from the Foretellers coming back,it's noted that an unspecified person(only thing we know about this person is that he's a guy) will also come back after Luxu's mission is completed as written in Secret Report 13,we don't know who this person is but as Chirithy,the Foretellers and Luxu consistently refers to MoM as the Master,the unknown person isn't him.
Epilogue spoilers:
Luxu regained the access rights to No Name.
The form Luxu retained after the second(?) death of his Nobody is that of Xigbar instead of Braig(Ultimania is vague whether he returned into being a complete person or faked his death).
Luxu awakens Ira,Aced,Invi,Gula.Ava,however isn't summoned from her sleep(because she's rejecting the summon?).
Ava's absence in the epilogue is supposedly because she successfully completed the task assigned to her by MoM(read:Dandelions),but the ultimania indicates this was only part of the reason she's absent(for the time being,she's represented as one of the 7 black pieces in the epilogue so she's inevitably coming back despite what Luxu said).
Maleficent and Pete witnessed the summoning of Ira,Aced,Invi and Gula.
It's inferred that the reawakening of the Foretellers involves the contents of the black box.
The chess pieces representing the new 7 Darknesses:
Luxu:Goat
Ira:Unicorn
Invi:Snake
Ava:Fox
Acedon:Bear
Gula:Leopard
Superbia:Heart symbol with angel and devil wings,the design of which is the same as the heart design in the Book of Prophecies,implying a connection with the individual and the book.
Secret Ending:
Sora ends up in (UG?) Shibuya after saving Kairi.
Riku ends up in Shinjuku/Verum Rex world
Yozora is seemingly a real person.
Master of Masters is also in Shibuya(because he's the only person kooky enough to form the heart sign on the moon while wearing the black coat).
Re:Mind stuff:
Luxord and Xigbar scene continues after Olympus's last scene,the former revealed he already talked to Xemnas about who ordered him to find the black box and questioned Xigbar about what's in the box,Xigbar lies about the box containing secrets of ancient keyblade wielders(which means the insides of the box isn't a copy of the BoP because Luxu was ordered not to tell anyone about what is in it) and alludes to his true identity,Luxord decides to play along and leaves while Xigbar questions Luxord's true identity.
Youngnort meets MoM at some point before his exam,the latter sent him on a trip around the worlds which made him realize that people are weak as a collective and that their self-rightheousness is just a front,concluding that the world is better off being ruled by darkness,MoM is being his usual self and foreshadowed Xehanort becoming powerful enough to control the darkness and told him his real name(which was censored so we don't learn about it)
Xehanort,Saix and Xigbar scene is just retreading what the Ultimania said about the True Org.
The price for is being sealed in the Final World and stripped of the Power of Waking so the taboo breaker can't leave the Final World
"Darkness" is revealed to be using Ven as a host and kicked out Sora from Ven's heart to Aqua's before he learns too much about Ven's memories
Lingering Will lost the fight against Terra-Xehanort which is why it was missing when Ven and Aqua fights Vanitas and Terra-Xehanort