Fi is descended from a long line of Lhao warriors. When he was young, his mother married Vogo Sun's father, and Fi quickly became friends with his new step-brother. To prove that they would always be brothers, Vogo branded the back of their hands with the Sun family's lion sigil, against later objections from their father.
Fi first appears when Kat objects to Vogo's claim of bad ore in the Banga Settlement's latest haul. Vogo orders his step-brother to make an example of Kat, but is stopped by Syd and his new mechanical arm.
Fi later returns when Kat challenges Vogo to a bet to force him to pass the Banga Settlement's ore shipment. He acts as Vogo's proxy in a target-striking contest, but when evidence of Vogo's cheating surfaces, Fi's pride as a martial artist demands a do-over, and he and Kat face off in a match of Bo'arack. When Kat wins the rematch, Fi forces Vogo to stand down, and the Settlement is victorious.
Later on, Fi appears in a number of side missions, such as a battle between Kat and a hundred of Fi's students, culminating in a battle against Fi himself in another Bo'arack match, and a mission involving the two of them working together to protect Vogo's cargo from rival shipping companies. When Lisa and several members of Banga are captured and held hostage by the council of Jirga Para Lhao, a remorseful Fi decides to watch over the settlement until her return, eventually helping Kat and company liberate the city and protect it from the unexpected return of Neu Hiraleon.
Fi is silent and reticent, yet firm and prideful, possessing a level of honor his step-brother lacked. As a martial artist, he always adheres firmly to his principles, even when such principles place him in opposition with his adoptive brother. Upon discovering Vogo's deceit in the target competition, Fi immediately demanded a rematch and challenged Kat to a game of Bo'arack.
Fi is an incredibly proficient martial artist and is quite physically capable. In addition to expert punches and kicks, he commands mastery of a yoyo-like weapon that can be used to strike over a wide area, and he can easily leap between the boats of the Banga Settlement with little difficulty.
The two are adoptive brothers. They cemented their bond at a young age by branding the backs of their hands with the Sun family lion crest, and true to their physical bond, the emotional bond between the brothers is very close, even if they may occasionally stand at odds with each other and hold disdain for each other's ways of life.
Fi respects Kat, both as a fellow fighter and as someone who is genuine and true to herself and to others.
- Fi's voice actor, KENN, is a well known Japanese voice actor and singer. He is best known for providing the voice of Jaden Yuki, the protagonist of the Yu-Gi-Oh! GX anime series.