If Cylian Sunsinger has left the battlefield by the time its ability resolves, its ability will affect each creature whose name is the same as the name Cylian Sunsinger had as it last existed on the battlefield.

When the ability resolves, it gives the +3/+3 bonus to the source of the ability and each other creature that shares a name with that source, even if that source isn't actually named Cylian Sunsinger. This could happen if Skill Borrower gains this ability, or if Cylian Sunsinger's name is changed (due to Mirrorweave, perhaps) before the ability resolves.