As a character, has he developed? Has he changed? Is he different to the way he was before?
At the start of the manga, Reiji was very clearly more optimistic than he is now. He had more faith in people and was more willing to be swept along by them, hence why he was coerced into having sex by two different people. Plus, he was more willing to open up and to actually discuss his problems with people and try to find ways out of them. Now, however, he's less likely to do that because he's seen the end results of where that takes him, especially with Miss Shiba, hence why he refuses to have sex with Chako and doesn't want her to save him, which is in complete contrast to how he acted when Miss Shiba brought him in. He explicitly said "If I'm going to be saved, I don't care who's it by," to "I don't want to be saved anymore." I think that's substantially difference in terms of his outlook towards how much he trusts other people, and where he's at mentally.
Begging the question a bit there. Also note his arc isn't over yet, and we're nearing a mid-act lowpoint I'd expect.
Regardless of whether people wanted to fulfil [sic] their own needs, people relied on him.
It's the issue of Kantian ethics, I'd say. If you live to fulfill the desires of everyone but yourself, measuring up to an impossible standard or increasing demands, then you'll be unfulfilled and be nothing more than a tool by those around you. If you treat people as a means to an end instead of an end on towards themselves, then it is inevitable they'll be demotivated, and find their lives miserable because they will have nothing to live for, which is why living purely altruistically doesn't work because the human experience requires some degree of selfishness or egocentrism for psychological purposes.
Maybe he sees that as pressure, and doesn't wanna live with that.
You're also neglecting that these people wanted him to do contradictory things and not for his own betterment, but to fulfill their own desires. Gen wants him to stay in the town because he loves him but can't be with him so strongarms him into a position where he can't go with force and blackmail. Miss Shiba wants him to leave with her because she wants to find someone to get married because of pressure from her family, and she's excused this desire away as being the embodiment of salvation for him. Chako wants hjm to attend the same university in Tokyo with him so they can be together, and his mother wants him there because she knows she can't take care of her mother's declining sanity and dementia and his brother's explosive anger. He can't fulfill all of their demands because they all ask different things of him, and don't care what he thinks. The closest we get to someone who isn't like this is Nagi, who's indifferent towards him instead of a guiding force, in everything but the most important question: to be or not to be?
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