His freaky, out-of-place nature was great, gave him a very eldritchian vibe.
Everything else about that character is a complete waste.
He says he has the special abilities of over 60'000 Hollows at his disposal, yet displays 3 or 4 in total. His Resurreccion is almost as bad as Zommari's, drastically reducing his mobility in a show where speed is the second most important thing after reiatsu (you need enough reiatsu to cut the enemy, then the fastest one wins, usually).
He was wasted into a weird therapy session for Rukia, never doing anything Kaien wouldn't have been able to do aside from turning into a giant octopus thing.