She is recruitable when you buy a dress for her at the Ice Lake, but if you dismiss her from your party before awakening White Rose, you will not be able to recruit her. After awakening White Rose, she is recruitable at the right room in the sleeping section of Leonid's Castle.
Asellus is specialized with Swords and Rapiers, her statuses say so, but as a half-mystic, she can transform into a mystic by use Mystic Glove, granting her the same benefit as mystic.
Asellus is just like Red or Fuse, unimpressive normally, but when she transforms, she becomes a machine of pure destruction.
Her cosplay item, the Asura boosts some of her statuses, making her a little stronger and losing her title as a unimpressive character to an very good character.