Oooh man hard question... Skoll was one of the characters that took MONTHS to figure out. He had a largely different backstory and was a tanuki zoan for a long time! I am very happy with how he came out design and personality wise, and his ship with Crocodile is wonderful. I was thinking about them and their daughter, Sophie, earlier today actually.
Tiramisu is obviously up there... I also do love Khazri (first OP oc I made) even tho I don't draw him much.

Writing... writing... I feel like a lot of the younger characters I struggle with because I just Don't Know what to do for them personality or story wise. A lot of them are confined to specific locations (lunarians especially) and I just don't rlly know what to do with em. S-Kite is a character whose personality I don't really have a solid grasp on, same with Rhoswen.
I feel like Akerue is a bit tricky at times because he's sympathetic... but also not? Writing him interacting with Crimson Peak (Den's Guild) is difficult bc Crimson literally take in outsiders- which Akerue is. Some sorta personal slight there, tho not sure what it was in canon (in an au Rose n I have, he caused Parvita's death).