マジパン

The phrases he says at the end:

"The clouds drifting in the sky will never stop"

"Can you see the stars shining above?"

"The wind whispers my name"

"Flightless bird, sing your song!"

And then Kai turns around like "The hell are you looking at?"

I was able to record this bit from one of the episodes. I looked through a G-Revolution OST playlist on YouTube but it didn't include all songs from the show, and I couldn't find it googling "kai sad piano" or anything similar either

Every single time Kai and Rei say each other's names (part 3 - G-Revolution)

Full compilation ↓

Every single time Kai and Rei say each other's names (part 2 - V-Force/2002)

Full compilation ↓

Every single time Kai and Rei say each other's names (part 1)

Full compilation ↓