Not sure how much of this would be considered spoilers so putting it in spoiler tags just in case. One of the things the WN constantly pointed out was that he didn't act well as a child. He didn't do a good job behaving like a child. The parents knew he was acting like a child so that they wouldn't divorce after Paul cheated on his wife. Paul realizes that Rudeus doesn't respect him as a father and they are closer to friends. That's actually important back when Paul thought Rudeus was bullying other kids when he was saving Sylphe because it was the explanation on while Paul jumped on Rudues, because he wanted to actually feel like a father that can discipline his child. The maid was always afraid of Rudeus at first because she also realizes that he doesn't act like a child. This is also why Paul expected so much from Rudeus when they met up, because to Paul, Rudeus was more of an equal than a child. Rudeus personality matches his appearance now because arguably, he always had the maturity of a teenager. He became a NEET since high school and stayed as one til he was 34. He didn't adapt as a child and he's not adapting as an adolescent. He was a grown man with the mentality of an adolescent to begin with. And like an adolescent, he's still maturing. As the series goes on, he'll learn how important family is. He currently has little attachment to his current family, something that he brought over from his old life. This is why he wasn't in a rush to save his mom. This actually gets brought up a lot, that because he was already an adult, he never got the child like attachment to his mom that everyone expects him to have. Thats why he is much closer to Paul, because he's like a friend. He'll Learn to be responsible for others. And by the end of the series, when he speaks with Hitogami and finally overcome his past, he is able to speak to Hitogami as Rudeus, and not his old fat self that he use to view himself as.