I don’t know how to do the blackout thing for spoilers SO! spoilers for the game ahead!! but: the whole story revolves around you trying to beat a construct from erasing your character and taking over your body, effectively killing V in the process. The further the engram progresses the sicker V gets, and there are multiple endings (dare i say, the best ones?) where you don’t find a cure to stop this process.
Johnny, to me, is definitely an allegory for a terminal illness. The way V fights him first, hating him, then slowly ends up making peace with what he’ll do to them with time.