Masaki Shido, known as Sid in Gunpla Battle Nexus Online (GBN), is a top-ranked independent Gunpla builder and pilot. He earned his status by overcoming numerous harsh Created missions without joining any force. His personality blends a deep passion for Gunpla with a self-directed nature, often following his own preferences. While occasionally blunt, he possesses a fundamentally caring spirit and readily helps those in need.

His primary Gundam in GBN is the MSF-007TE Gundam Tertium. During his explorations, Masaki met Cuadorn, a sacred beast from Eldora researching ancient technology to fight the antagonist Alus. Masaki agreed to aid Cuadorn against Alus's forces. Alus captured Masaki, forcibly converting him into "The Masked Man," a brainwashed agent piloting the MSF-007SS Gundam Seltsam. This transformation trapped his consciousness on Eldora, causing his real-world body to fall into a coma roughly six months before Hiroto and the Build Divers arrived.

As the Masked Man, Masaki fought under Alus's control against his former allies. His consciousness remained imprisoned on Eldora until the Build Divers, led by Hiroto Kuga, discovered his plight and executed a rescue mission. They successfully freed Masaki from Alus's influence, enabling him to awaken from his coma. He recovered under the care of his sister, who had tended to him throughout his unconsciousness.

News of Masaki's ordeal spread across GBN after his return. Build Divers member Kazami created a video detailing the rescue mission and the threat of Alus. The video gained massive traction, exceeding 10 million views and inspiring countless Divers to offer strategies and support against Alus and his army, demonstrating the widespread impact of Masaki's story.