there's still a lot that can be brainstormed, but i was thinking he was this person's imaginary friend at a young age and they either sort of grew up together because back then lucca looked like a child himself or he was envisioned to be like the person's protector, and he stayed around until they aged out of that phase. only, lucca at that point had been created, so he didn't simply vanish but rather continued to exist on an astral/other plane. when exactly they tapped into their powers and brought him back is also something we can figure out, but i was thinking the power itself was probably along the lines of summoning, drawing creation, or something to do with astral manipulation? however that pans out, lucca gets to be a "real" person, and their friendship gets a bit strained and co-dependent because he knows a lot of their secrets/fears and he's really only alive as long as they want him to be, but the two of them do have a very strong bond, so they walk a very frayed line of friendship and sometimes complicated resentment.