Solo is the last descendant of the ancient tribe of Mu, as well as the human form of Rogue. He has a Wizard named Laplace, but does not use him to EM Wave Change. He can wave change on his own, and is registered under Project TC as No. 002. Solo's exact origins differ vastly between the games and the anime.
In the third game, he is also less hostile than before, but tells MegaMan that neither that fact nor his assistance changes anything.
MegaMan Star Force 2
He was orphaned as a child, and people often banded together to attack him during his life, making him a very solitary person.
Solo says that people have always been afraid of him, and that they often ganged-up on him and rejected him as a freak. He can EM Wave Change because he draws his power from the lonely voids in people's hearts, essentially loneliness waves. He is hostile to Geo because he is afraid that Geo and his friends will come together to harm him (but they really have no intention of doing so).
Dr. Vega says that Solo is so obsessed with reviving Le Mu on his own that it goes to the point of him thinking of not working alone, while Hyde says that either way, he won't trust him. He denied Hollow's offer to use the Mu matter transporter, "Kamikakushi (or the Indie Proof)," saying his blood will not allow others to help him.
Near the end of the second game, Solo begins to have a change of heart. He saves the unconscious Geo from the falling continent of Mu, though when Geo speaks to him, Solo hides it with a response that "Your unconscious body was blocking my way out." This is a single, momentary and rather unhelpful change, as Solo later challenges Geo to a final showdown (as a secret boss, after you complete the game) to prove whether or not his power of loneliness is stronger than Geo's link power. He teleports Geo there sometime after the defeat of Le Mu (although it is unknown when, although it could have been a few seconds or a few months later, although the game skips straight to it after the credits), and asks him for a fight. (You can't save at this part of the game, but you do have the opportunity to walk around the surrounding area and rearrange your Battle Cards)
MegaMan Star Force 3
In MegaMan Star Force 3, Solo becomes more of an ally than an enemy, though he does still come and go as he wishes. Once Geo encounters him, it is revealed that he has gained his own combat Wizard, Laplace.
Solo later reappears at Echo Ridge Elementary in class 5-A, where he insists on waiting for Jack Corvus to strike in an attempt to draw out MegaMan. When this does happen, Solo wave changes into Rogue and assists MegaMan in fighting their way to the roof, but challenges MegaMan himself just before they reach it. MegaMan defeats Rogue and moves on to defeat Jack Corvus. After this defeat, Solo can be found waiting to challenge Geo at the Hibiscus flowers in front of Alohaha Castle.
Much later, just before MegaMan attempts to enter the Black Hole Server, Solo challenges MegaMan as Rogue once again, only to lose once more. It is revealed then that Sirius holds an item called the Mu Metal, a very rare item that he plans to augment the Laplace blade with. However, this would require waiting for a while longer, time enough for the black hole to swallow Planet FM.
Unfortunately for Solo, MegaMan defeats Sirius, who transfers the Mu Metal directly into him. This prompts Solo to hone the power of Noise and challenge MegaMan to a final showdown in the Meteor G Control Cyber Core as Rogue ZZ. No matter how many times he is defeated, Solo still despises the idea of bonds but he does admit that Geo is strong enough to protect them.
The last known sighting of Solo is during the credits for the third game, where he is seen in a grayscale picture, alongside Laplace and in a desert-like area similar to that of Whazzap
In the anime Ryuusei no Rockman Tribe, Solo works on his own, seeking the OOPArts to fulfill his duty to prevent the reappearance of the Mu continent and, along with it, the reawakening of Le Mu. He appears infrequently during the search for the OOPArts, always seeking to challenge MegaMan, though he never says a word to MegaMan.
When MegaMan, Harp Note, and Cancer Bubble travel to the Mu continent once Dr. Vega has opened it, they find Solo somewhere in the ruins in some form of suspended animation. When his stasis is disturbed, Solo awakens partially wounded, and reveals to the three that his only friends had perished to seal Le Mu.
Despite his injuries, Solo wave changes into Rogue and begins to fight Hollow in an attempt to stop Dr. Vega from resurrecting Le Mu. Though he loses the fight to Hollow, MegaMan arrives and interferes with the OOPArts, prompting Hollow to switch priorities and go after him.
The last time Solo is seen is when Le Mu awakens and attempts to widen the portal between Mu and Earth. As Rogue, Solo appears before Le Mu, and attempts to hold him back while he instructs MegaMan to seal him. Reluctant at first, MegaMan uses the OOPArts to seal the portal to Mu, Le Mu, and Solo with them. His final fate is unknown.