Since I've seen a lot of fan content that only showcases Kaldur as an eternally stoic and level headed individual in any given situation, my personal HC is that he is highly competitive but a bit of a sore loser when it comes to game nights. He always loves a good challenge, strategy games being an all time favourite of his because of how good he is at it and the opportunity it provides for flexing his strategy skills, but can get really pissed when he loses (especially if the teammate who won is a big shit talker).
I would like to imagine one game night where they are playing rummy he thinks he is on a winning streak until the tables turn unexpectedly and he loses hard. He just gets up abruptly, thanks everyone for playing and then leaves without further acknowledgement, just to meditate underwater. I would like to imagine Rummy is one of his favourite games so the meditation session goes on for longer than usual because of how upset he is. But the anger still persists, evolving into giving the teammate who beat him the silent treatment for two days straight (unless they need to talk for a mission).
Eventually, he apologises, feeling guilty for letting his emotions get the better of him knowing that it could cost him and the team greatly if it were to get out of hand as per his Atlantian training. He relays that the anger was unsightly and he won't be participating in game night again. The team uses this opportunity to be vulnerable with him, sharing details of moments they have regretted (at least as much as they can for privacy reasons). They also assure him there's no need to feel guilt for having negative emotions as long as he doesn't do anything brash (unless its using wally as a punching bag /j).
Now he feels slightly more comfortable being vulnerable around his teammates. He still gets pissed when he loses during game night (sometimes the silent treatment lasts longer) but he can handle his teammates teasing him for it knowing they aren't hurt by his anger and can find the absurdity of it. The team does not share this information with the Justice League to preserve Kaldur's reputation as the "well behaved responsible" one and spares him the embarrassment (Kaldur still tells Arthur anyway and they have a heart to heart)
Personally, I love hcs that give Kaldur a bit of a pissy side or an ego. Because of his role as team leader, he feels as if he needs to be the emotionally closed off member for the sake of his teammates. I can imagine, putting on an air of unwavering stoicism when emotional vulnerability can be seen as dangerous has caused him a lot of stress.
I would really love to see more fics about him where he is allowed to be himself outside of vigilante work.
