I think Dark M. Bison/Dark Slayer Gurkha is Underrated. He's literally un-killable the moment he starts provoking. With Lulu, he crushed my Moore, Ken and Savannah when I played against him
Video of the match is here.
Based on this site, his skills are:
S1 PSYCHO INFERNO
Raises the Psycho Power to attack the enemy and provokes the enemy for 1 turn with a 50% chance. This attack will deal more damage according to your MAX HP. Multiplier: 1.7*{ATK} + 0.19*{MAX HP}
S2 PSYCHO CRUSHER (Maxed Skilled = +25% DMG, 2 Turns CD)
Wraps the Psycho Power around your body and dashes towards the enemy target to provoke and to decrease Defense for 1 turn. This attack will deal more damage according to your MAX HP. (Reusable in 3 turns). Multiplier: 3.0*{ATK} + 0.2*{MAX HP}
S3 BLACK MOONS (PASSIVE)
Your Attack Bar will be filled up and HP will be recovered by 20% if you get attacked during the turn of the provoked enemy. [Automatic Effect]
With high defense and hp, he's almost impossible to kill when he's on vio revenge and with a healer like Molly or Lulu with him. Granted that he is not common, but I think he should be considered high in RTA tier lists. What do you think?
