This means how I see their relationship right? Where do I even start...

So I imagine Fischl is very awkward around him unless she manages to keep her persona up around him, which is SUPER hard to do for her because she gets all shaky around him due to how much she looks up to him. Both of them are, in fact. Diluc because he doesn't know how to act around her, especially in public where everyone is looking.

When Diluc wants to seriously speak with her about anything, he has to say her real name which causes her persona to IMMEDIATELY fall apart and she becomes super awkward and unable to make eye contact. Something that btw always happens when she asks him to princess carry her (she does that a lot, that brat! It was one of the inspirations for the wedding art). In fact one of my favorite HCs is that they both avoid eye contact as much as possible, but whenever they DO look at each other, it's when she needs something from him, so she pulls his collar down forcing him to look down at her, and when their eyes connect it's the most intense staring contest ever, their eye contact is so infrequent that whenever Fischl tries breaking eye contact first he holds her face still, and- !!!!!!! even better when she takes her eyepatch off... (special thanks to oomf Elmo)

Also yeah he was very annoyed at her larping at first but he eventually caved in and gives her a little fanservice when they're alone because he knows how happy it makes her to see her always serious bf act all cool and edgy just for her. He cringes every time but if it makes her smile for a moment...!

I also HC them as you'd expect, the 16 y/o teen with the 23 y/o or so adult (idgaf!) Laws in Teyvat are built different what can I say.

I could entertain the father/daughter HC (in the lewd way wink wink). I'd say they fit the DDLG perfectly, I'm giving even the familial HC people what they want!!! Sugar daddy and her little princess, this is what you wanted!!!