Thorn is the leader of the Ostinato Musicians.
Thorn is a lovely young girl with a sickly appearance. She appears before the protagonist's party in an uncanny, phantasmal way -- she always shows up where she couldn't possibly be.
She is the oldest and first composer of µ's songs, known for her enormously popular dark and brooding love songs.
µ trusts Thorn completely and obediently follows her orders.