The Claire seen in the Moon That Never Sets is the younger version of Haschel's daughter and Dart's mother. She switches between a state possessed by the War God and an innocent state of her own consciousness.
The player must fight her with Haschel on the Moon That Never Sets. At her current youthful state she doesn't recognize her own son Dart as stated by Haschel before he goes to meet her in combat.
Haschel's long lost daughter who disappeared after accidentally killing her best friend during combat training in frustrated response to Haschel's harsh words. She later in life met Zieg, Rose's former lover, and married him. When the Black Monster came to Neet looking for the Moon Child, Claire went back to help the other villagers alongside Zieg, but perished. The Claire that Haschel must fight is younger than she was when she left home. She is possessed by the spirit of the War God, the spirit that Haschel has sought to attune himself with through his Rouge Art.
Claire is a challenging boss, but not too difficult. She has one special attack, Four Gods Destruction. Her first set of attacks are a simple set of physical attacks. After about 3/5 through the battle, she uses her special. This attack will reduce Haschel only to 1 each time. She uses this once or twice then continues with the physical attacks, but from here she does two or three attacks in a row, so after the round of special; use blocks to heal yourself.
Though it is not stated directly in the game that this Claire is both Haschel's daughter as well as Dart's mother, it is shown to be all but a certainty when Haschel discovers Dart knew the same tune as his daughter, who had said she would teach it to her child one day. Evidently Haschel believes it to be true, too, when he says to Dart, "she does not know you yet" (the Claire on the Moon is a younger one, who has not yet left home and borne Dart as a baby). He knows of the connection but Dart remains oblivious.