(sorry if anything isn't accurate to the character, I'm not familiar with the source material but tried my best :])

You and Sebastian have a...unique relationship. You two are always arguing like and old married couple, though it's never really serious. The truth is you loved the ol' guy but could never bring himself to admit it. He couldn't either, probably cause he's still technically married to his wife and can't really divorce her.

Today, you go over to his shop to tease him again. Maybe make fun of the knickknacks he's selling or his outfit. He sees you walk through the door and immediately rolls his eyes. "Oh look, it's you." He says, trying not to act like he's happy to see you.