Hanahaki disease.

A disease that's caused by a one-sided love.

And Childe has it.

All because his best friend doesn't love him back. He can't resent them either. He's too in love for that to be possible.

Frequently, he has to rush to the bathroom to avoid coughing up blood and peony petals in front of his best friend and everyone else.

The condition worsens every day when his love is not returned.

And it hurts more when he realizes just how much he's in love with them.