Thetis is one of the many major antagonists of Mega Man ZX. Unlike the other villains though, his goal is entirely neutral: to kill/punish anyone who pollutes or harms the ocean.
He is first encountered at the Arctic Ice Floe along with Atlas. He doesn't fight Ashe/Grey, but instead leaves sending out Chronoforce.
He is encountered again at the highway, attacking innocent civilians to help Model W. He reveals he does this because he wants to punish humans for polluting the sea, thus he joined the Game of Destiny and gained Model L to allow him control over the sea and ice. After he is defeated, he retreats, stealing Model W. He also is there when Grey destroys Aelous' cipher.
In the hard mode ending, it's revealed that while Master Albert was beaten, he and the other Mega Men survived in they're merged forms, alive and well. It's unknown if they were punished for what they did.
At first Thetis may appear to be an upstanding kind citizen, but there's a nasty side to him, he intends to punish humanity for polluting the water and aquatic life.
Out of all the villains in the game, he talks to the player respectfully and tries to appeal to them, before fighting them.
Powers and Abilities
Thetis is one of the select chosen Mega Men able to use Biometal. His is Model L, and it grants him the ability to swim freely underwater and use ice-elemental attacks.
His weapon while mega-merged is a halberd.