In another world, Ace probably wouldn't have survived his extensive injuries. Hell, even now it was a close thing. An impossibly close thing.
Somehow, working together, Marco and Law (and now you know why his moniker is the way it is) were able to... put him back together. Fix Ace-s impossibly broken body. However, even the near miraculous power of both devil fruits combined wasn't an immediate fix.
So almost a week later, still in one of the impossibly stuffy rooms of the Polar Tang, you're still sitting at Ace's bedside waiting for him. Well, sitting is generous. You're on the chair, but slumped forward so your head is pillowed on your arms resting on the bed. Ace hasn't so much as twitched for days, so it's startling when there us suddenly a hand in your hair.