(I'm sorry this is just one of my top ships and they are CUTE-)
-Your choice if they are together or not for the scenario-
Of course, everyone was insecure about something. Whether it be their body or the way they act. There's always something.
Tsukishima? Tsukishima was insecure about a patch of skin on his back. It was large and splotchy and he didn't like it one bit.
And Yamaguchi knew of this. He's managed to get a few glances of it and at some point, Tsukishima had admitted to it when he was practically half asleep. Yamaguchi wasn't even sure if he remembered telling him.
It wasn't until they were having yet another sleepover (they practically live at each other's houses) when Tsukishima was wearing a loose shirt. It fell at his shoulders and revealed a bit of his back along with most of the patch of skin.
They were cuddled up with Yamaguchi sitting behind him and some movie playing. Yamaguchi started to lightly trace the patch before giving it a quick kiss. In all honesty, he found it beautiful.
(You can uhh... switch the roles if you wanted. I just want wholesome)