Yes this is another fairly limitless one being as how the kids all have very fascinating lives and if you apply their respective alpha selves as guardians you obviously end up with a lot of food for thought. The alpha kids all touch on their lives with their guardians a little so we have plenty to work with but I can't bother rereading again so I'll elaborate on my ideas for him with what I remember.
In canon they spend no time together if I recall correctly Dirk just is aware of what's happened in the world he lives in or something which I'm going to twist around for this. Entertain absent Alpha Bro with me but now consider he is at least around enough for Dirk to get insight into his life and for Alpha Bro to build a dynamic with >15 year old dirk.
Obviously Alpha Bro was very preoccupied with attempting to save the world and whatever else fighting clowns fighting aliens making movies taking bumps of coke plotting with Alpha Mom so on so forth. Yet we all know Dirk is quite a big fan of his bro. All of this build-up to say that while Dave does care for Dirk, if they interact he is likely to be emotionally abusive. He uses stimulant substances, another common head canon but I agree, he's generally more sensitive than the others as a kid so it's easy to assume he would be somewhat emotional as an adult. Especially assuming he's still raised by his bro and now has this strange "I see him in you" situation on top of not having a very prime example on how to parent better, which he wants to do but does not have the ability to.
What easier way to avoid responsibility of the end result of Dirk than avoiding him as often as possible? Maybe this can be elaborated on in another post, but imagine how that makes Dirk feel hahaha... Poor kid. Learned from the best, in an infinite loop! Ay Caramba! When they do interact, Dave tends to require support. He's sloppy, he's been worn thin, he's going to die young and he knows this, and Dirk while disturbed at times sees this as his opportunity to build rapport with his idol/bro and makes measly attempts at caretaking him.
Alpha bro rejects this assistance when he's coherent enough to do so but sometimes he uses these moments as opportunities to apologize to Dirk, in long... nonsensical, drunken spiels... Some words make sense and are particularly hefty for his vocabulary, so Dirk infers his bro has likely sought advice from Rose, the famous author, and Bro only then would be attempting to repent for his wrongdoings while shoehorning her lingo into his paragraphs. Among other things, of course.
