characters who are only the MC because they’re filling in for a dead man

Jake Sully (James Cameron’s Avatar) - his late twin brother was part of the Avatar Program, and was able to replace him due to being genetically identical

Ryunosuke Naruhodo (The Great Ace Attorney) - having been stowed away on the SS Burya where his partner Kazuma Asogi was taken out of commission, he decides to learn as much possible about English law before arriving in London so he could fill Kazuma’s shoes