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. In addition, his intelligence was such that he could craft fallback plans in case his initial plans did not work. This was especially evident in the Last Story, where Mephiles proceeded to use a Chaos Emerald to kill Sonic, after Silver evidently stopped trying to kill Sonic.

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.

It has also been speculated that Mephiles might also be somewhat of a coward, judging from several actions he displayed throughout the game:

Instead of fighting Sonic face-to-face, he appears behind him without Sonic or Elise knowing, and proceeds to impale him from behind. In battle, the action of attacking from behind is usually an act of cowardice.

In addition to this, he creates a distraction to keep either of them from noticing him while he proceeded to kill Sonic.

During both boss fights, Mephiles hardly attacks on his own without aid, either by summoning shadows, or cloning himself to use as distractions. He uses the shadows in the first boss fight as shields, and increases the number of them when you get too close.

During the boss fights, he waits to attack Shadow until his back is turned.

He also doesn't fight Shadow directly until the player expels him from Shadow's shadow.

In the second boss fight, you cannot face him until you clear the floor of the seething shade covering it, having to use Chaos Boost to expel him.

He managed to convince Silver and Blaze into attempting to kill Sonic before actually doing it himself, which would in turn release Iblis, though there was a much stronger form of Iblis already free in his time while he had a Chaos Emerald at hand.

In the time ten years previous to the story that takes place, when Shadow corners the newly created Mephiles before sealing him away, Mephiles' voice sounds startled if not frightened before quickly shifting into his more recognizable tone as he makes a threat to Shadow.

Despite this, however, he has demonstrated some fortitude, such as when he willingly let Omega beat him into submission and even laughed when the latter intensified in his attacks after he implied Omega's role in sealing away Shadow in the future.

Mephiles is voiced by Dan Green, here is an example of his voice: https://www.youtube.com/watch?v=k1ixdJSoDe8