Garnet Til Alexandros XVII is the princess of Alexandria and daughter to Queen Brahne. Thinking that something is off with her mother, she is "kidnapped" by the Tantalus crew, and adopts the Dagger persona to hide her identity. She is initially naive to the outside world, but soon becomes accustomed to it, showing off some street smarts at times. She enjoys the theater, with Lord Avon's I Want To Be Your Canary being her favorite play.
Dagger's main ability is White Magic, essentially making her one of two healers that can join your group. Unfortunately, she doesn't learn every single White Magic spell, so she might be considered the lesser of the two. Her other unique skill, Summon, allows her to call Eidolons, which are pretty similar to summons from previous games. Unlike the other character, Dagger gains access to more Eidolons, so she is better suited for that aspect once you have both characters in your party. Note that the Eidolon's full animation won't always play, but Dagger's Trance changes Summon to Eidolon, giving it an increased chance to play out the full animation (for more power). Also, while in Trance and after using the Eidolon command, you will get the shorter summons throughout the battle, which don't take MP or the Trance bar.