Not completely wasted, but REALLY half-baked - Kenjaku from Jujujtsu Kaisen. I relly like the concept of mad scientist who lived for so long that to satisfy his scientific curiosity he's ready to doom all world. My main problem with him it's that plot doesn't really need him after start of culling games. Below there's things that could make him better character.
1) Just give him a time to opposite Yuji like with Mahito and Sukuna. Otherwise, it's just strange that despite being Yuji's creator, they barely interact with each other. It's like Luke would never know what Vader is his father.
2) Explain a little bit more why he became like that (not necessarily give him tragic backstory or something - just make him tell, for example, what he doesn't care about people and world as whole because he lived for too long time).
3) Also give him idealogical foil. Mahito has Yuji. Sukuna has Gojo (and Yuji). Kenjaku doesn't really have something like that.
Because of lack anything i described - he feels more like plot-device rather than character
