Yuu Kajima was there from the start—a Federation pilot at the Battle of Loum who stumbled onto a mostly intact Zaku in a colony during late U.C. 0078. That discovery ended up being crucial for the Federation's V Project. As leader of the Marmot Team, an experimental unit testing new tech before it hits the broader military, Yuu got his first real taste of combat when his squad ran into the notoriously uncontrollable RX-79BD-1 GM Blue Destiny Unit 1. The thing tore through Philip and Samana before Yuu barely managed to drive it off. Later, when a Zeon suit that fought just like the blue GM came after them, the GM itself showed up and saved their hides—a weird moment that wouldn't be his last encounter with that machine.
Yuu was soon assigned to pilot the Blue Destiny himself. He took to it fast, though the EXAM System messed with his head pretty badly. The system's true nature came out during a mission over California Base: Marion Welch, a young Newtype girl, had been trapped inside the machine after a failed experiment by her boss, Chlust Morses. Her consciousness wound up imprinted on EXAM, and she'd been trying to reach pilots through it—usually driving them insane in the process. Marion begged Yuu to destroy the system and free her. When he faced off against Zeon ace Nimbus Schterzen piloting the MS-08TX(EXAM) Efreet Custom, both pilots and their EXAM Systems went down together.
The real conspiracy emerged soon after. Morses turned out to be a paranoid xenophobe convinced Newtypes would dominate humanity. He'd built EXAM to keep non-Newtypes on top. Before Yuu could confront him over the newly completed Blue Destiny Unit 2 and Unit 3, Zeon special forces hit the base. Nimbus hijacked Unit 2 and killed Morses, then bolted for space. Yuu chased him to Side 5 in Blue Destiny Unit 3, and the two clashed one final time. Nimbus went down for good, though Yuu drifted in space before his team picked him up.
Yuu's reputation as "Blue Death" stuck with him through the rest of the One Year War and beyond. He stayed active through the Gryps Conflict, both Neo Zeon movements, and eventually joined Londo Bell. He was flying an RGM-89 Jegan when the Federation pushed Axis back at the end of Char's Counterattack.
Content compiled by AnimeList.moe from publicly available sources.
