An old (very old) lady, though what kind of creature she is exactly is unknown. She resides in a dimensional vortex, inbetween the Home and Another dimensions. When you first meet her, she helps Serge come to terms with what's going on, and gets him going again in his mission. Sprigg can transform into various enemies and use their attacks in battle. To aquire more forms, see Tech Skills below.

Recruitment
Sprigg joins a bit after you meet her. Since she's required, there's no trick to getting her.