The Way You Used To Do
"We're really sorry," his father says, in a teary-eyed, wobbly way. "But your friend, Izuku, he's... He's gone, son." Katsuki can do nothing but blink up at them for moments that feel like an eternity, eyes darting between both his parents in obvious confusion, disbelief, and, more than anything, indignation. "What the fuck are you two talking about? The damn nerd is standing right beside you!"
During a battle, Midoriya gets hit by a villain whose quirk detaches his soul from his body. Stuck in a ghost-like state, the boy enters a race against time in order to save himself from permanently dying. Much to his luck - or lack of it -, the only person who can see and talk to him in this state is no one other than Kacchan.
Alternatively: Deku and Kacchan are soulbound.
