Here is the full list of all 20 Viola achievements.
Beat the first level.
Recruit a full party.
Recruit all 10 characters.
Learn Viola's first song.
Learn all nine songs.
Buy something in the Catshop.
Reach Support Level S with Fenrys.
Reach Support Level S with Niko.
Reach Support Level S with Soleil.
Reach Support Level S with Momo.
Reach Support Level S with Sybil.
Reach Support Level S with Emerald.
Reach Support Level S with Mallow.
Reach Support Level S with Ceryn.
Reach Support Level S with Arti.
Reach Support Level S with Justice.
Viola remains in Vylna, never returning home.
Viola returns home, destroying the world of Vylna.
Viola retains the connection to Vylna, forging her own path.
Complete every Personal Quest.