Fei is a character with two faces: one as the chill Tai Chi practicing fighter with friends throughout Purgatory. And the psychotically obsessed Tiger's Vessel and proud disciple of Tiger Niko.
I still believe there are unresolved plot points regarding Fei's life and upbringing. How/why did he learn Tai Chi, and did the Worm have something to do with this? Misasa speculates on why this martial art is unknown, but it is deliberately cut off. During Tokita Niko's exposition, I believe we will see more scenes with a younger Fei.
The most apparent traits of Fei are his playfulness, cockiness, and pride (as THE Tiger's Vessel). But what is also cool about Fei is that underneath his Worm/Tiger's Vessel facade, I believe he is a decent person at heart. I do not think his friendships with other Purgatory gladiators were actually fake, and I believe he is lying to himself and Tokita Niko to impress Niko. He also respected Purgatory's rules and didn't kill Waka when he could have countless times. Why should he care about Purgatory or Purgatory rules when his role is the Tiger's Vessel?
Yet he respects Purgatory rules enough to try and ring out Wakatsuki, even when he was damaging his life with Divine Demon. He even asks Wakatsuki to surrender so he wouldn't have to kill him.
After Fei pushes himself too far and is laying on the ground, on the verge of death, his final moments are telling. He sacrificed his whole life trying to please Tokita Niko and become the Tiger's Vessel, thinking Niko cared about him in return. Yet in typical Tokita Niko fashion, he was manipulated into believing he was an extraordinary prodigy of martial arts- much like Lu Tian was.
Fei is humble and congratulatory in death. This is his "Purgatory gladiator" personality where he was friendly and fit into society.
However, when he finds out he was manipulated and lied to by Tokita Niko, and that he was never intended to be THE Tiger's Vessel, his entire identity shatters. You see him reduced to a pathetic state, which is intentional. He is another pointless casualty by the horrific other Tokita Niko.
I wish more of the fight was spent characterizing Fei, as I believe Sandrovich was very subtle with a lot of these character details. Yet when you piece them together, you have a remarkably complex and interesting antagonist in Fei. It's a shame he has to die like this, but I believe that we are approaching the Endgame for the Worm plotline. At this point, we have the Other Tokita Niko, Eddie Wu, the boss Yan, the #2 of the Worm, and Tokita Ohma/Gaoh Ryuki.