Thetis

Thetis is a character from Mega Man ZX Advent. He is a kind boy that loves swimming and the sealife, but he wants to punish and rid people from the world for dumping their waste into the sea. He is the chosen one for Model L, the Ice Mega Man. The player battles him at the Highway.

Personality
He's a kind person who appears to be an upstanding person. He loves the ocean and sealife. Beneath this exterior is an extremist however. He harbors animosity and hatred against mankind, participating in the Game of Destiny in order to gain the power to kill all humans and restore the earth to its pure state.

Mega Man ZX Advent
The player encounter him at the Arctic Ice Floe with Atlas. Instead of fighting you, he flees leaving you to fight Chronoforce.

The player encounter him again at the Highway where he is attacking citizens to feed Model W. He Megamerges and jumps into the ocean with you following him. He explains that he is participating in the Game of Destiny to punish humans for polluting the sea. Then you battle him and defeat him. He retreats and takes the Model W with him. He appears briefly after you break Aeolus's cipher with all the other Mega Men. He also appears after the defeat of the eight Pseudoroids in Ouroborous with Atlas, Aeolus, and Siarnaq. They are about to attack Grey/Ashe when Vent/Aile arrive and fights them instead, allowing the player to continue. After defeat of Albert, Ouroborous starts collapsing. Model Z seperates from Vent/Aile in order to keep Thetis and the other Mega Men busy. Model Z freezes their Biometal which freezes them. Their fates are unknown, but if the game is cleared in Hard Mode, you can see all the Mega Men well and alive in their Megamerged forms. (It's also implied that the original immortal Reploid Data was inserted into them, as, right after Master Thomas asked Master Mikhail what he think happened to the original immortal Reploid data, they showed up).

Attacks
When you fight him, you're in the ocean, reminiscent of Leviathan's battlefield in the Mega Man Zero series. He dash-swims in the screen, attacks and then swim-dashes out of the screen, sometimes even feinting his attack. When stopping, he twirls the Halberd around and fires off a triangular wave of energy not unlike Childre Inarabitta. His charged attack differs on who you are fighting him as. As Grey, he fires off a pair of ice dragons that home in on you. As Ashe, he manifests a thick, spiky ice formation which he then shatters, releasing a series of ice shards. These attacks can be countered using a fire attack or the Homing Lasers.

His ultimate attack involves him summoning a machine from underground that shoots out several snowflakes and then proceeds to create a vacuum that sucks in all nearby, the snowflakes themselves been dispersed about from the force. They can be destroyed before the vacuum effect occurs using Homing Lasers.

Human or Reploid?
Thetis is somewhat of a controversial character, as his species is contradicted. The fact that he has a upside down red triangle birthmark on his forehead would imply that he is a Reploid. However, in Grey's storyline, Grey explicitly refers to Thetis as a Human.