Waluigi

True Form. When Waluigi drops to half hp or less, he enters his true form. While in his true form, he gains the ability to use his Multiattack twice per turn, his Fist and Kick attacks deal 8 bludgeoning damage, he gains the ability to use 'WAH!', and he takes 1 less damage from all attacks. 

The Ultimate Underdog (1/Day). When Waluigi drops to zero hp, he does not die. Instead, he regains 10 hit points, and gains the 'Super Saiyan' trait, along with the ability to use 'Wilting Rose' and 'Big Racket.'

 Super Saiyan. Waluigi channels all his energy into an empowering aura. Waluigi can no longer be targeted for opportunity attacks, gains a flying speed of 80ft, and all attacks against him deal 2 less damage.

Multiattack. Waluigi makes three attacks. If he has them available, he may replace an attack with 'WAH!' or 'Wilting Rose.'

Fist. Melee Unarmed Attack: +3 to hit, reach 5ft., one target. Hit: 4 bludgeoning damage.

Kick. Melee Unarmed Attack: +3 to hit, reach 10ft., one target. Hit: 4 bludgeoning damage.

Whip. Melee Weapon Attack: +10 to hit, reach 10ft., one target. Hit: 6 (1d4 + 4) slashing damage.

Spin Kick (Recharge 4-6). Melee Unarmed Attack: +3 to hit., 5ft., area directly around Waluigi, up to eight targets. Hit: 4 bludgeoning damage.

WAH! (True Form Only, Recharge 5-6). All creatures within 30ft of Waluigi must make an Intelligence saving throw, DC 15, or take 35 (10d6) psychic damage, and be stunned until the end of their next turn.

Wilting Rose (1/Day, Super Saiyan Only). Waluigi unleashes a devastating blast shaped like a rose blossom. All creatures within 30ft of Waluigi must make a Dexterity saving throw, DC 18, or get hit with four Whip attacks. On a successful save, they only get hit with one.

Tennis Racket. When Waluigi would be hit by any ranged attack, he may add 3 to his AC. If this causes the attack to miss, he fires it back at the attacker, using their attack and damage rolls.

Big Racket (True Form Only). When Waluigi would be hit by any ranged attack, he may add 6 to his AC. If this causes the attack to miss, he fires it back at the attacker, using their attack and damage rolls, however, the attack also knocks them prone if it hits.