More of the “Chara likes to pretend to be Frisk’s older sibling” headcanon. I can see them trying to force the kid sibling/older sibling dynamic on Frisk at first, as a way to manipulate and control them, but over time, Chara kind of forgets why they’re being so nice and protective of their vessel, and looking after Frisk and trying to make the journey through the underground as bearable as possible just becomes routine.
They’re probably singing one of Toriel’s old lullabies.
