Koashura is a demon from the Spirit World who serves as the primary antagonist of the first Yu Yu Hakusho animated film. His actions are driven by a deeply personal grudge against Koenma, the son of the Spirit World ruler. Koashura and Koenma were classmates in Spirit World kindergarten. Koashura developed feelings for a girl named Kotennyo, but his attempt to confess his love was cruelly interrupted by Koenma, who humiliated him and presented the flowers to the girl himself. This slight fueled a burning desire for revenge.

To exact his vengeance, Koashura teams up with a powerful demon named Garuga. Together, they kidnap Koenma while he is on vacation and demand the Golden Seal, the symbol of Lord Enma's authority, as ransom. Unbeknownst to Koashura, Garuga is actually the notorious demon Yasha in disguise, a figure who is manipulating him to secure the Golden Seal for his own ambitious purposes. This betrayal places Koashura in the complex position of being both a villain and a pawn.

Koashura initially presents himself as a bitter and vengeful schemer. However, his characterization gains depth following his betrayal by Yasha. When Yasha is defeated and killed, Koashura reveals that his true intentions were never to cause a large-scale catastrophe. He was driven purely by a desire for personal retribution against Koenma for the emotional wound inflicted on him in their youth, and he ultimately shows that he did not truly wish to hurt anyone.

As a demon, Koashura possesses notable combat abilities, with a particular strength in aerial attacks against his opponents. He engages the protagonists in battle directly but is ultimately overcome. His defeat brings an end to the kidnapping plot, concluding his tragic cycle of revenge.