Chaos Kyrieloid (Ep 5 + 30 MP), Ultraman Agul V2 (Ep 7 + 20 MP), EX Tyrant (Ep 8 + 40 MP), Bullton (Ep 1 + 10 MP).
You don't necessarily need to win every battle with an "S" Rank to unlock the base characters, but aiming for high ranks helps unlock costume variations and gallery art. ultraman fighting evolution rebirth unlock all characters