Leaf

Leaf is a red fairy in The Legend of Zelda: Phantom Hourglass. He is the Spirit of Power, and is held on the Isle of Ember in the Temple of Fire until Link rescues him. His direct role in the plot is fairly short; he opens the door to the basement of the Temple of the Ocean King, explains how Phantoms work, and has some other expositionary dialogue, but otherwise mostly only acts alongside the other spirits. Along with them, Leaf helps Link steer to the Ghost Ship through its fog bank.

By leveling up Leaf at the Island of Spirits, Link can equip him and make his sword twice as strong (accompanied by a flame effect). Doing so again doubles it further. This is the only way to make the sword hit harder, as Oshus's Sword and the Phantom Sword are the same strength. Additionally, Fire Bubbles and Fire Keese can be attacked directly without needing to be stunned using him, and Oshus's Sword can stun Phantoms by hitting them in the back.