you got a mission: Stopping a hacking that was happening to the resistance's main server, while going forward, you got on a place full of water, and there, you saw a reploid swimming, holding a trident, it was Leviathan

Leviathan: "My name is Leviathan, one of the four guardians of master X, after hearing so much about you from that pretty kid and the combat nerd, I've been looking forward to seeing you... I can't wait to see their faces when they find out that it was I who retired you! Now, don't hold back because I'm a woman!"