Fai is first shown sealing away King Ashura of Celes Country at the bottom of a pool. He transforms his creation Chii (originally from Chobits) into a net-like shield, instructing her to alert him if King Ashura should awake. He then uses his magic to transport himself through dimensions, to the shop of the Dimension Witch, Yuuko. Out of his traveling companions, Fai's motives for joining the group are the least clear. He is the only one to arrive at Yuuko's Shop purposefully, by use of his own means. His wish is to never return to Celes country, but as the price for this would have been too great, so he travels worlds with the group instead. He fears King Ashura’s awakening, believing King Ashura would chase him through different worlds.
From Syaoran's group of allies, Fai is the most energetic, while being enigmatic at the same time. He appears to be happy-go-lucky and good-natured, and acts very carefree. He often teases Kurogane, calling him ridiculous nicknames such as "Kuro-rin" and "Kuro-tan". However, Fai has been shown to be as perceptive and a skilled fighter as Kurogane. He also appears to carry a deep emotional burden, and values his own life very little. In battle, Fai doesn't bother to fight very hard for his life, and will only do so if someone he cares for is in danger. Though he hides his unhappiness well, Kurogane is able to sense the disguise. When he confronts him about it, Fai usually gives very dark and enigmatic answers. His past is a profound motivating factor for his actions and is something Fai cannot seem to overcome; instead he masks it all with a smile.
Fai is emotionally distant, refusing to allow anyone to become too close to him. Despite this, Kurogane has noted that the mage has become attached to Syaoran and Sakura. Fai's behavior changes significantly after he becomes a vampire and is forced to regularly drink Kurogane's blood to survive. Fai becomes much colder and very solemn around everyone, and begins addressing Kurogane by his real name in order to try to kill the bond between them, confessing to Yuko that he had 'crossed the line' that he had drawn for himself and become too close to Kurogane. However, after the events in Celes, Fai seems to have gone back to the way he was at the beginning of the series, as he once again addresses Kurogane with strange nicknames, and he seems to give a real smile now.
Fai worries about his companions and would go as far as breaking his own personal vows to ensure their safety. Kurogane confronts Fai about this, and states that Fai should figure out what he's doing with his present and future, because his past has nothing to do with either Kurogane or their younger companions. Fai tells Kurogane that he should not worry or care about him, which Kurogane responds with "I don't care about your past. So prepare your current self." After Kurogane leaves, Fai collapses, emotionally exhausted, and states that this is too hard for him.