In your opinion, what is the most fascinating demon boss that isn't a major character?

For me, it's Phantom. I love spiders due to a consistent interest in Spider-Man and with Phantom holding the distinction of the first boss in the series, the game left an undeniable impression on me afterward. With as many times as he's fought, I'd almost argue he should be a major character. While I'm aware of why he wasn't a familiar in DMC5, it still would have been cool if he got some sort of representation or boss theme remix in that game. That said, what non-major demon boss do you find most fascinating?