Lesser Devil vs Greater Demon

[Mihoshika]

Is there some reason for there to be the greater demon, which actually grants you less benefit than the lesser devil, which you also get at a lower level?

I mean, aside from the minor (relative) boost you'd get from doing so when you're splitting familiar levels multiple ways.

[EvilGandhi13]

Lesser Demon is a flat additive bonus, Greater Demon is percentage based. The short answer is that the LD boost is only good early game, where it's definitely better. If your stats are a couple hundred (the exact math will vary based on what your other additive stat boosts are), GD becomes better, and after a few Reincarnations LD won't be noticeable at all.