So a Xuile'solen works as a spy, a hidden bodyguard, or a limited assassin, perhaps. But he's gonna need a little help. Well, for one, he's blind - good for being sneaky and not being snuck up on, bad for coordinating neutral colored attire or handling jobs requiring a bit of sight to get things done.
So, perhaps his employer/clan pushes an apprentice on him, or he somehow finds a kid tagging along that nags him to the point of insanity, and he figures "better teach him/her something so they don't bungle my jobs clinging to me like a dingleberry." Maybe.
Enter Tu'suust - partner and apprentice to Sjaalven. She's young and a bit spunky, but takes her new lot in life seriously. Perhaps in the beginning she had some issues with her teacher's ugly mug, but over time it has just become something normal, expected. He might be a hard teacher, but she takes it in stride.
She also makes sure he doesn't walk out wearing bright, crazy colors. Unless she's pulling a prank.