Fairy Leviathan, commander of Neo Arcadia’s Deep Sea Squadron and underwater specialist. Later as Model L, biomatched with Thetis (second picture) and in Biometal form (third). She has something of a playful attitude, but can be vicious, especially towards Zero.

Leviathan is tricky to match up to any of X’s physical Armor attributes, because as far as I know (and I may be wrong) there are no particular Armors with specialty aquatic characteristics. There is, however, a manga-only character from the X series who is a mermaid Reploid, which is the closest thing to Leviathan (Marty, in the fifth image). This doesn’t work with the overall Guardian origin though, so we have to assume that there is an aquatic Armor that Light built and X used, but not in any game we’ve played.

There is, however, a close parallel in MM9 in Splash Woman (fourth image), one of the very few female Robot Masters (there’s actually 3 or 4… kinda). She’s a mermaid with a weapon very similar to Leviathan, and her deisgn has some similarites too. So if Tornado Man is Harpuia’s counterpart, Splash Woman is obviously Leviathan’s (being the only female Guardian, and water-aligned). So Light, having built her, must have gone on to create Armors for X with her strengths. He must have found and used them at some point, which has not yet been represented in a game. There. That works.

More importantly, there is a very strong tradition of Copy Abilities with an Ice or Water theme. It is these weapons which went towards the formation of Leviathan when X fragmented.