There's an isekai fantasy manga where the main character (mc) is reincarnated into a world where there are multiple reincarnators all trying to defeat/kill the demon king (dk), because a deity promised to give the person that kills the dk one wish (I think. Might be thinking of another manga).
Anyway, the mc gets a skill steal ability and meets a guy that has (I think) a super growth ability. But because the super growth guy is innocent/ignorant to the ways of the world, the mc decides to protect that innocence/ignorance.
(As a R18 manga, I think I remember the mc fornicating with a receptionist because he thought she had bad intentions and, spoiler,
I think she did.)