Mephiles has three forms; the first and original was a dark, blackish-purple formless ooze or gas with some gaseous qualities. In his second form, Mephiles appears almost identical to Shadow the Hedgehog, aside from having green eyes, no visible mouth, and the fact that everything that is red on Shadow is gray on him. His third form (and his preferred one when he uses his abilities) was a crystalline, anthropomorphic hedgehog form resembling the appearance of other hedgehogs in the Sonic series, such as Sonic himself; in this form, he has no visible nose, mouth, or feet. His name was based on the one of the names of Mephistopheles related to the legend of Faust.
Mephiles was accidently created during the Solaris Project, under the supervision of the Duke of Soleanna. He was actually an extension of Solaris himself; his conscious part. He was sealed in the "Scepter of Darkness" by Shadow the Hedgehog, who had traveled through time to ten years before the events of the game. In the present, Dr. Eggman accidentally releases him while trying to steal the scepter from Shadow and Rouge, and runs away scared, not even bothering to send his robots to attack him. Mephiles then transformed into his Shadow form, and sends Shadow and Rouge into the future after he realizes that Shadow doesn't know who he is. He seeks to get the Chaos Emeralds and bring pain and despair to Princess Elise, with the goals of making her cry, therefore releasing the "Flames of Disaster" (a.k.a. Iblis) and then fusing with him, to reunite them both as a single entity; the original Solaris.
To accomplish this, he tells Silver the Hedgehog and Blaze the Cat that Sonic the Hedgehog was the cause of Iblis being released, which led to the destruction of their world. Sonic was, thus, referred to as the "Iblis Trigger". He sends the two back in time, making them believe that killing Sonic would save the future.
Along the way, as all three storylines progress, the three main characters discover how Mephiles was somehow involved in the incident which created the "Flames of Disaster", wielded by Princess Elise. Mephiles has a very pivotal role in all of the storylines, throughout the course of the game. He has the most direct role in Shadow's Story, where Mephiles is shown to hold a particular hate for Shadow since it was he who sealed him in the Scepter of Darkness. He also has a major role in Silver's Story, as he manipulates Silver and Blaze into attempting to kill Sonic so that he can finally fuse with Iblis. He has a very minor role in Sonic's Story, where he is only seen when Sonic, Knuckles, and Tails are eavesdropping on his conversation with Silver and Blaze that ends with them time-traveling back to present-day Soleanna. Mephiles attempts to break Shadow's will to fight and join him, much like Black Doom had, by revealing that in the current future, the world, and an unwilling Omega, would turn against Shadow and imprison him in suspended animation (Mephiles keeps his hatred for Shadow a secret), attempting to get Shadow to join him, saying they would both get revenge on the world. However, Shadow proved immune to Mephiles' tactics, saying that he would determine his own destiny, leading to a battle between the two of them. Though Shadow takes the upper-hand, Mephiles knocks him into a wall with an energy blast and incapacitates him when he lowers his guard, but Omega intervenes and drives Mephiles off. Later, Omega meets Mephiles again, who reveals to Omega that he was one that would imprison Shadow in the future, leading to the robot shooting him up with two miniguns. As Shadow (along with Silver after Shadow foiled his second assassination attempt on Sonic) had traveled back into the past at that point, he learned of Mephiles' true nature (and Silver learned that Mephiles had lied about Sonic being the Iblis Trigger). He, along with Omega and Rouge, later meet him again. Again, Mephiles offers Shadow a chance to join him, but Shadow, knowing Mephiles' true intentions, flat-out refused the offer. Mephiles is fought again (he's the final boss in Shadow's story), and loses. After his seeming defeat, however, Mephies proves immune to being imprisoned in a new Scepter of Darkness that Shadow tried to use on him, saying that it wouldn't work on him anymore (he implies that the reason he was immune was due to his time traveling to ten years ago, due to his line of "The "I" of now absorbed your powers through your shadow in the past."). Mephiles then uses two Chaos Emeralds to create an army of clones of himself. He then boasts to Shadow that he can't defeat him, and asks why he is even bothering to protect a world that will eventually turn against him. Shadow responds by saying, "If the whole world decides to become my enemy, then I will fight like I always have", and then, mirroring a similar ability from the Sonic Adventure 2 adaption of Sonic X, takes off his bracelets and easily defeats the entire army.
In the Last Story, Mephiles, having either escaped or simply survived the fight with Shadow, finds the purple Chaos Emerald, uses it to warp to Sonic and Elise's location shortly after they escape Eggman's exploding Egg Carrier, blinds them with the Emerald's powers, and impales Sonic through the back with an energy beam, killing him instantly. Elise cries over his death and, as a result, releases Iblis, allowing Mephiles to fuse with him and transform into Solaris. It is also revealed in the Last Story that his lie to Silver and Blaze about Sonic being the Iblis Trigger was actually a half-truth: Sonic is indeed the Iblis Trigger, but his death would not stop Iblis's resurrection; in fact, his death would be the very event that would release Iblis from Elise's soul. The key to Iblis's freedom was an event that would cause Elise to become so consumed with despair that she would cry, and she had formed a very close (possibly even romantic) bond with Sonic.
Due to Sonic and Elise's actions in the past, Mephiles never existed.
Many of Mephiles's offensive techniques are based on that of past Sonic bosses. He can sink into the ground and become the shadow of his opponent (particularly Shadow's since you only fight him as Shadow, but on one level you fight Mephiles as Omega); this was not unlike King Boom Boo. He can create massive numbers of Shadow-like creatures, but they are very easy to kill. Once using two, he created an army of clones. He can also create massive spheres of black energy that home in on their targets. Mephiles's final attack used before transforming into his Solaris form was a long energy bolt (dubbed the "Dark Chaos Lance" by fans) that he extends from his right hand to impale Sonic.
Mephiles appears to be immortal, surviving every fight he takes part in, and whenever he is beaten he seems to just shift into his ooze-like form. Whether or not he can feel pain is debatable, since he seems to react to taking damage when he serves as a boss, but when he met Omega in Wave Ocean, he just stood and let himself get shot multiple times; if he did feel any kind of pain, he certainly didn't show it.
Mephiles is also a highly skilled manipulator, as he was able to effortlessly manipulate eleven people (Sonic, Tails, Knuckles, Amy, Shadow, Rouge, Omega, Silver, Blaze, Eggman, and Elise) across a time period of 210 years (from the failure of the Solaris Project to the post-apocalyptic world of Silver's future) in order to get a single result in the present. However, this also spawns the question of whether Mephiles travels through time after Silver to do this or simply exists through all the events, making alterations in the future that he believes caused events in the past, to stay the dominant force. Nonetheless, Mephiles can teleport himself and others across vast distances and through time, not needing a Chaos Emerald to do so. Shadow mentions that this aspect of Mephiles's powers feels similar to Chaos Control; though whether or not he actually does use Chaos Control is unknown, it does seem likely: Black Doom mentioned in Shadow the Hedgehog that the Chaos Emeralds are not really needed to induce Chaos Control, but their powers amplify the effects. If he does indeed use Chaos Control, it must be more powerful than Shadow's, as Sonic, Shadow, and Silver needed to perform a "Cross-Chaos Control" with two Emeralds in order to time-travel, while Mephiles can travel through time at will without even one Emerald.
Mephiles has also demonstrated the curious ability to, with one Chaos Emerald in his possession, warp the other six emeralds to his location instantly, which he did after the Flames of Disaster were released from Elise and before reuniting with Iblis.
- Mephiles' name derives from Mephistopheles, one of the chief demons of the Christian faith.
- Mephiles was the only villain in the various Sonic continuities to canonically kill Sonic, and the first villain in the game series to have killed someone on-screen.
- Mephiles was voiced by Dan Green, the same actor who voices Knuckles the Echidna. He is somewhat famous for sounding almost exactly like Atem/Yami Yugi from Yu-Gi-Oh! and Silva from Shaman King whom are also voiced by Dan Green. In Sonic the Hedgehog, he is credited as his alias, "Jay Snyder".
- Mephiles seems to have a lot in common with Shadow, like how both were results of science experiments to help mankind, both were attacked by members of GUN (in Mephiles' case, it was Shadow, who worked for GUN at the time), and so on, though Mephiles, unlike Shadow, was truly evil.
- Mephiles appears on a card in Sonic Rivals 2, after earning an S on knockout mode.
- Dr. Eggman had shown an interest in capturing Mephiles, trying to take the Scepter of Darkness, and later when Shadow asked Eggman who Mephiles was, he said he would tell him if he brought him Mephiles. It isn't stated why Eggman wanted to capture Mephiles, but based on his actions in the game, it can be guessed that he had hoped to harness Mephiles' powers to travel through time.
- Mephiles is one of the characters (along with Fang the Sniper, Tails, Amy Rose ,and Bean the Dynamite) to have a title that is not based off their own species; that is because Mephiles is not of a species, unless you count "darkness" as his species.
- Even though Mephiles appears to lack a visible mouth, the general area on his "muzzle" moves in and out as he speaks in a manner similar to an inflating balloon.