Anything can be yandere if you put your mind into it.
Toothbrush? Yandere. You can't use any other toothbrush, even by accident. Television? Yandere. Don't look at anything else. The sun? Yandere. Even the moon can't hide you.
A ghost is untouchable, unpredictable, and unfortunately, inevitable. You are the one who took their life, after all.
Yandere!Ghost who quite literally can't do anything to you.
They can't feel you, touch you, cuddle you, move anything, kill anything, or even breathe. Sure, they can protect you from other ghosts, but it isn't the same as the living. It irks them to know they can't enjoy you to your fullest potential.
If we're going in the comedic route, they're going to be a 24/7 nagging ghost. They constantly whisper into your ear—or shout loudly, or in between—about things that don't really matter.
Like, why the hell are they telling you to avoid your best friend? They can't do anything about it anyway, even if they're choking their ghastly head off.
Just because you're the one who killed them doesn't mean you'll do them any favors.
That brings us to the darker route. Let's say you're guilty about killing them.
They will use that fact against you. In this version, they're still whispering into your ear, but you're actually affected by it. The yan!ghost preys on your fear, insecurity, and guilt.
You're like a pawn on the board. Heart filled with nothing but regret, sorrow, and despair. With the vile need to make up your mistake: their death.
You killed them.
In one way or another, you must make up for your sins.