Solo is the Lone Soldier (Kokou no Senshi) and sole survivor of Mu. He constantly battles with Rockman in order to obtain his OOPArt - the Sword of Berserk. He has a mysterious power to Denpa-Henkan using his loneliness in order to become Burai - Rockman's nemesis during this season. Burai is formidable with both a sword and with martial arts, making him a very tough foe.
Solo was sealed so he could destroy Ra Mu if it was ever resurrected. His true motive for gathering the OOPArts were to keep Ra Mu from ever being reborn. Orihime gathered all three OOPArts and managed to resurrect Ra Mu and Burai - thanks to Rockman's help - was finally able to destroy it. Solo disappeared inside the Mu portal in order to seal Ra Mu.