Gai Yuuki, aged 25, defined the words "obnoxious", "independant", "quarrelsome"... He practically made his career out of it. He loved to fight. Finding himself a Jetman, expected to be a member of a team and desperately needed, well that was an experience for him, too.
Like all of them, he was a member because he had received the power. He was not happy about it, but he did come through for the team, every time. Fighting with Gai gave Ryu something solid to distract himself from the agony of Rie's death. Fighting with Ryu gave Gai something to pretend he was there just to annoy and not because he wanted to be.
Gai fell in love with Kaori. It wasn't love at first sight, more like love at first slap. She appealed to him in part for her gentleness and class, for her spirit and bravery which he recognized, and also because unlike almost every woman he met, she would not be his for the taking. He began to put himself at risk to protect her, and it quickly became an addiction. One particular time he took a shot from a deadly new Vyram, and soon found crystals bursting from his flesh. Eventually he believed he was dying, and fled from his fellow Jetman. Kaori eventually found him. He was trying to be brave, but the agony got the better of him. Then when he thought he could feel himself changing, he swore his love to Kaori, then tried to flee beyond her sight. She had to watch him turn into a giant crystal. When they managed to destroy the new Vyram, everyone it had turned to crystal returned to their human state, and Gai was able to come, just in time to help his team-mates completely defeat the Vyram.
A second time he nearly died for her, and Kaori admitted to herself as well as him that she loved him, too. They became an item, but not for terribly long. For all his obsession with her, and all the good qualities both had, they were very much different from each other, very unsuited. Gai broke up with Kaori when she tried to make him over as a businessman for meeting her parents, rather than him getting to be presented as himself.
Parted, though ever in each other's thoughts (it's hard to throw away an obsession), they continued to support their teammates. While he and Kaori were still together, Gai had learned about Ryu's fiance, Rie, and had to watch a man he'd come to believe in fall to pieces. He finally accepted his place in the Jetman as a sort of de facto second leader. It's hard to realize, when you've spent most of your life fighting to be anything but what people force you to be, that your teammates regard you as a hero and that is what you want to be. Eventually Gai met Gure in one-on-one challenge, and after a severe battle, a lucky shot saved him. Honoring his opponent's request for a last smoke, he lit the cigar Gure carried. Then he came just when he was needed, and together with his teammates defeated the last of the Vyram, Radiege.
Three years later, buying flowers for his beloved friends' wedding, he stopped a mugger and was knifed for his troubles. Believing he was going to die, this time he did not hide from his friends, but made it to their wedding too late for the ceremony. He met Ryu and wished him well, gave Kaori his blessings from a distance, as he was too weak to get up. Probably he died. But I think he was at peace with himself.