Yuji is revealed to be the son of Kenjaku who took the body of his mother Kaori so that he could create a vessel for Sukuna. This is why he is called brother by Choso and is able to use blood manipulation techniques. Nothing he does improves or develops his character. He feels guilt over Sukuna killing people during the Shibuya Incident and wants to kill himself, but his grandfather and the character motivations that are half baked but present in the first half of the story are nowhere to be found. He's a generic good guy who feels really bad but does nothing wrong and is super special with no understandable character motivations. The author tries to resist formulaic shonen bullshit but exemplifies it and has no other way to resolve the relationship between Yuji and Sukuna other than for Yuji to pull out a surprise domain expansion that is literally Talk no Justu.
Nobara is hinted to be alive after being fatally injured by Mahito, but doesn't return until the Shinjuku Showdown arc. And when she does return it is only for one small part of the fight with no explanation given or any further character development. That one moment is her Sakura moment where she is a girl boss and uses her resonance to do barely anything about Sukuna. She doesn't have a character arc at all. Her arc ends at the end of the Shibuya Incident arc.
Megumi doesn't have any character development either and becomes a one dimensional character fixated on protecting his sister until Sukuna tricks Yuji and transfers himself to Megumi. After this Megumi is literally not a character at all and has no impact on the story because he becomes depressed and can't take control back from Sukuna. Even when Yuji breaks through to him later in the Shinjuku Showdown arc he just lays on the ground and cries inside of his own body, not even slightly inconveniencing Sukuna. The author intentionally wrote him out of the story by doing a special ritual to suppress Megumi and having Sukuna kill the incarnated body of his sister with his own body to break him.
Gojo is not developed any further either and the author hates his character. He isn't unsealed until the Shinjuku Showdown arc. Gojo loses to Sukuna because Sukuna pulls out a bullshit attack at the end of the fight to cut Gojo in half. For some reason none of the main cast is bothered by this and there is no appropriate fear or reaction. Everyone just moves on until the author teases that he was actually alive. They introduce this black box about how Tengen secretely knew that if Gojo left the prison realm there was a condition and binding pact that would kill him, and Kenjaku is worried over this. Don't worry, nothing comes of it. The tease was actually because Okkotsu made this secret plan to copy Kenjaku's technique so he can transfer his brain into Gojo's body to use the Six Eyes and Limitless techniques against Sukuna. He manages to do nothing against Sukuna and quickly leaves the fight.
Kenjaku's motivations are that he wanted to enact this stupid plan to merge everyone into Tengen so he could control it and see what happens. He gets killed because of a throwaway gag character with an overpower cursed technique that allows him to bend reality into whatever he believes is funny. Okkotsu is there to finish him off since the gag character doesn't kill anybody. After getting jobbed. he says that someone will inherit his will, and nothing comes of that statement at all. Because he got jobbed everyone blames Okkotsu for having to be the one to finish him off since he was delayed in returning and taking Gojo's body.
Sukuna's backstory references a massive plot during the Heian era (mostly to give him a retarded form change midway through the fight with the sorcerers during the Shinjuku Showdown arc) that is not explored in-depth at all. They name drop a bunch of shit that the author doesn't explain in the story and just moves on from. It's not explaineed very well why the Heian era was super powerful, it just is. It's a time period to pull powerful incarnated sorcerers from that have a one-dimensional relationship to Sukuna to generate fake mystery. Gege pulled all of this out of his ass to rush out this hackjob.
Yuki Tsukomo is built up as a major supporting character, but gets killed before even releasing her domain expansion against Kenjaku. She has no character development other than mentioning that she was a former platinum star vessel for Tengen as a jab at her. THIS IS NEVER EXPLAINED. Her connection to Toudou is never explained. In the flashback she is connected to Geto, but Geto's character has nothing to do with Kenjaku. They have different motivations and interests.
Instead of focusing on the important side character (and main characters) the story during the Culling Game arc focuses on introducing random characters like Hikari and Higurama that have no wider impact on the story. There is an entire story about a French guy who is a manga artist that get's jobbed by Hikari's bullshit cursed technique which gives him infinite cursed energy and makes him immortal. Kenjaku convinces America to send troops to capture sorcerers so he could kill them and use the soldiers as fuel to generate cursed energy, so there is this drawn out sequence of him talking the president and it's like playing a knock-off Kojima game without any of the charm. As an American I was on the floor about how embarassing and retarded that scene is, and it was based off a throw away line by Yuki at the end of Shibuya Incident arc.
The entire last arc is one long drawn out fight against Sukuna where every character no matter how powerful gets jobbed and has these built up final attacks before Sukuna pulls out another hidden technique or gimmick. Even after fighting Gojo he is revealed to have not been at full power at all and manages to have infinite asspull cursed energy while the tension throughout the fight quickly wears down. This is over the course of dozens of chapters of him having the most plot armor out of any character in the series. Few of the characters die and so it became a revolving door of a character being introduced before jobbing and then being replaced by another character because Sukuna is a retarded character. I laughed out loud when they brought out the magical black man and the gay guy who were followers of Geto to fight Sukuna before leaving shortly after midlly inconveniencing him. The Shinjuku Showdown arc is the down-syndrome short bus version of the last arc of Naruto.
The ending resolves none of these character conflicts, answers none of the remaining questions about the world or the story, and ends with a slice of life about Megumi, Yuji, and Nobara continuing to do missions as normal.
