Nam-Gyu and Se-Mi sibling dynamic
I love the idea of Nam-Gyu and Se-Mi being siblings (or even twins - much to each other's dismay), with Se-Mi HATING the fact that Thanos just HAS to come to Christmas dinner with their family. Like, imagine that Thanos and Nam-Gyu have known each other for a while, started dating in high-school and Nam-Gyu's family is fine with him coming around because they've been used to it for the last 5~ years. So why not invite him over the Christmas dinner with the family? Not much could go wrong, right?
Well, Nam-Gyu wants to (obviously) sit with Thanos, so Se-Mi uses this seating plan to her advantage, doing anything to annoy Nam-Gyu, and plops herself right next to the poor oblivious Thanos with a smile when everybody settles down.
She'd smile lazily, fiddling with her hair and asking Thanos questions, just to get under Nam-Gyu's skin. When he eventually shoots her a "totally not obvious" glare she just keeps going, asking Thanos about himself or trying to get him to reveal anything about Thanos and Nam-Gyu's dating life etc. etc.
Nam-Gyu is a very private person around his family, so she wants all the details and poor Thanos doesn't know how to say no to her. When she asks a question that particularly touches a nerve and makes Nam-Gyu's eye twitch, the two get into an argument at the table, which causes a scene.
Thanos is between them with the paper crown from his Christmas cracker lopsided on his purple spiky hair looking into his lap and wondering how he got into this situation
After the party and dinner is over Nam-Gyu takes Thanos off somewhere private and profusely apologises, saying he'll make sure Se-Mi doesn't do it again. Thanos just shrugs and acts relaxed about it, he was incredibly stressed in the moment but he wanted to do whatever it took to win over Nam-Gyu's family, so he would deal with Se-Mi if it meant he could come back for another dinner.
Se-Mi eventually grabs Thanos from a conversation and reluctantly apologises, not because she wants to but because she feels she has to with Nam-Gyu glaring daggers into her back at every turn. She ends the conversation with a pat on Thanos' back and then whispers "you're a good fit for him" or something adjacent "but if you tell him I told you that, your guts become kebab meat, got it?" Thanos gulps and nods nervously.
Nam-Gyu pesters Thanos about what she said for the remainder of the night, but he won't give.