Mystraven is the 2nd house that you can join, oriented primarily for rogues, or "trickers, pranksters, or relaxed people". Their logo is seemingly a raven, and their house leader is Casca.
Mystraven Mechas and Weapons tend to be unpredictable at best. Although they can deal some of the highest damage outputs in the game (Their basic Mecha has the potential to do over 120 in one hit) they can also fall flat on their face when your luck runs out. Still, assuming you've invested alot into your "Luck" stat, Mystraven Mechas and Weapons can truly amaze you.
Their only mission is currently a quest to fetch a trans-dimensional mirror, thus irritating the Runehawk house. Upon its completion, the Labirynth opens up. It is generally considered one of the best places to go to for levelling purposes. Their Lesser Knuckle Duster can deal Lacerating Strikes like Sys-Zero's Symbolic Blade.
DON'T try to complete the first Advanced Combat Theory class with it, because the Runehawk deflects critical hits, and since Mystraven mechas depends on critical hits to deal damage, using a Mystraven is NOT a good idea.