What would you say is Caelus's best quality?
The fact he takes so many things seriously.
His conversations, the inner thoughts about inanimate objects (and their life stories), throwing himself into every little act he does, etc etc. To him life is something meant to be celebrated, and that certainly means no half-assing on that route. So don't let his way of being 'dismissive' at times fool you.
Now this isn't be confused with his scale of investment. While he'll take something dumb as hell seriously, it doesn't mean that it'll be the thing to 'pierce the heart' so to speak. It just means that he'll give it the due respect it deserves and remedy it accordingly. Unless it's fun, then he might just join you on that.
So in a lot of ways it also leans that he'll be genuine when it comes to the intertwining of life stories. What you're into, how you approach life, it's all details he like etching to memory. Due to being a Nameless, it's a lot more open to strangers as well, not just his friends or companions.
It also opens the door in how measures of authority will naturally butt heads with him. If it's for someone's day to get better, he won't hesitate to jump through those loops (within sensible reason) to have that come to light. Even if it means high profile people like Herta may be pissed at him.
Which, fair. She should be if she isn't busy worming her way into something else.
But yeah, that's the gist of it.