Masaki Shido, operating as Sid within Gunpla Battle Nexus Online (GBN), is an independent S-Rank Diver renowned for his extraordinary Gunpla building and piloting prowess. He achieved top-rank status by tackling demanding missions under severe conditions without force affiliation.

Before his coma, Masaki exhibited a free-spirited, caring nature as a Gunpla enthusiast, often aiding others despite occasional bluntness. His involvement in Eldora commenced when he agreed to assist Cuadorn, an ancient entity opposing the antagonist Alus. During this mission, Alus captured Masaki and forcibly converted him into "The Masked Man," piloting the MSF-007SS Gundam Seltsam. This process trapped his consciousness within Eldora, leaving his physical body comatose for roughly six months.

His sister, Mizuki Shido, cared for his comatose body throughout this period. The Build Divers, led by Hiroto Kuga, discovered Masaki's plight and prioritized rescuing his consciousness during their Eldora mission. Following his rescue, Masaki allied directly with Cuadorn to confront Alus. In the final battle, he deliberately provoked Alus into attacking GBN, enabling a coalition of Divers to ambush and defeat Alus's forces. This action resolved the Eldora conflict and permanently severed Alus's control over Masaki.

Post-rescue, Masaki's consciousness returned to his body, ending his coma.