I think the key moment comes back in 3-5...Kevin smiled. "Not necessarily. He loves you, you love him,
or the song wouldn't have worked."
"What about you?" she wondered. "You were singing too."
"Balance," Kevin replied. "Am I not of the Choir of Elohim?"
Rei nodded, but Kevin hadn't met her eyes when he said it.
Like the bossman said, he's not singing because of balance. He's singing because he loves Jon, too.