Mephiles has three forms. His original form, which is also his base, is an amorphous mass of darkness. In his second form, Mephiles appeared almost identical to Shadow the Hedgehog, aside from having green eyes with slit pupils, no mouth (though his muzzle moves when he speaks, as if he does have a mouth), and the fact that everything that is red on Shadow is a pale powder blue on Mephiles. His third and most common form was a crystalline, anthropomorphic hedgehog with no visible mouth, nose, or feet. His eyes also change color in this form, gaining a red sclera and lighter green irises.
Unlike most of the villains in the Sonic series, who are driven by a lust for power or revenge, Mephiles (according to Shadow) only seems to crave destruction. He is portrayed as a cold, manipulative, cruel, sadistic, and ruthless villain with no sense of compassion. He would do anything to accomplish his goals, even if it means taking innocent lives. Mephiles happens to take great pleasure and delight in hurting others and has been shown to have a very warped and twisted sense of humor, making him a very sadistic and psychotic individual. For example, he made it his sole mission to cause pain to Elise throughout the entire course of the game, and even broke out into a maniacal laughter after ending Sonic's life. This is also seen when he taunts Omega by telling him he would be reprogrammed to capture Shadow. Mephiles is also highly intelligent, capable of using and manipulating people to further his evil plans. This is shown when he preyed upon Silver's frustration and desire to save the world, by deceiving him into thinking that Sonic was responsible for the devastation of his future, knowing that killing Sonic would have done the exact opposite of what Silver was trying to achieve.
Mephiles also has nihilistic tendencies, as evidenced in his final fight against Shadow, where he asks why Shadow even bothers fighting at all, and why Shadow even bothers defending humanity when they will inevitably persecute him later. However, it seems that he has some manners, as he bowed to Shadow after taking his form, though this was most likely done as a mocking gesture.