I’m having Feelings about the very different ways that WWX plays up his childishness around his adoptive siblings.
Like with Yanli, it’s more obvious, and intentionally so. When he acts childish around her, it’s both more of a game - like playing house with your siblings as a little kid, “you can be the mom and I’ll be the baby,” or like over the top “I’m Baby” jokes - and more genuine in that it reflects on his actual need, even as a young adult, to receive care and comfort in ways that reassure him that someone sees him as precious and worth fussing over. He lost his parents, was homeless as a young child, was emotionally and at times physically abused by his adoptive mother, had to live around the brokenness of his adoptive parents’ marriage — he has very real reasons to have abandonment issues and trouble feeling like he even *deserves* to be safe, loved, and looked after. Yanli’s Oldest Daughter Syndrome / innate Mom Friend tendencies / genuine kindness as a person provide for a serious psychological need that he has, and as a much older sibling with a strong nurturing streak, she also obviously thrives on being able to mother him a bit. To her, his immaturity, even when it’s not a thing he’s playing up as a joke, is an endearing sort of vulnerability, and it draws them close as she protects him.
With Jiang Cheng, it’s much more subtle. It’s not a game they’re both in on, it’s an act that Jiang Cheng is meant to buy into.
WWX ignores or pretends to ignore a lot of things, but he’s very perceptive about people as a rule. He knows from an early age that it’s... awkward, to put it mildly, for Jiang Cheng as his father’s heir to have an adoptive brother who’s so talented and so clearly favored by his father, even to the point of being his clan’s head disciple at a very young age (and to a point of sparking rumors that he might be the Jiang siblings’ bastard biological half-brother). As his mother frequently and cruelly points out, WWX’s over the top talent and his favor in Jiang Fengmian’s eyes can easily be turned into a *threat* to her son.
So what does he do? He makes every effort to cede the Older Brother Role to his technically-slightly-younger brother. It’s partly genuine, sure, because he IS genuinely a pretty carefree guy, but it’s also really clear when you’re looking for it that he plays up being a flighty good-for-nothing so that Jiang Cheng can have the power of being the one who has to guide him and keep him out of trouble. It’s a way of saying “you’re not just the Legitimate Heir, I’m basically the silly baby brother anyway so it doesn’t matter if your father favors me, or even if people think I’m his Secret Love Child, because you’re the grown-up one and I’m not a threat to you or your position.”
And in ways, for a long time, it probably worked. It clearly helped inspire protectiveness in JC, initially to shield him from their mother’s abuse when he could and even in some ways long after they fell out. It was a way he could get JC to relax and play around more, too, something he clearly needed as a kid. But it also cemented him in JC’s mind as someone reckless, genuinely immature, thoughtless, irresponsible. It made it so that when WWX had to start looking at his life and making difficult, painful moral choices that pulled them away from each other, JC couldn’t even truly entertain the possibility of him being *right* about any of their disagreements — it was obviously just WWX being impulsive and not considering other people because he’s a selfish goddamn manchild and that’s what he does, so why wouldn’t he just listen to him and come back to the people who loved him and needed him???
It’s just painful to think that a coping mechanism that he was initially able to use to bond with JC and make him feel secure ended up playing such a role in making it impossible for JC to trust him or believe in him when they both needed to be able to rely on each other the most, with such painful consequences for both of them (and also like, thousands of people who got Necromancied to death, I guess.)