Tsukasa has always been bad at math.. so he got a tutor. they were almost the same age. Rui was one year older than him. Tsukasa tried to solve the math task what Rui gave him, but apparently Tsukasa couldn't do it. Rui chuckled and decided to give him motivation..

"Come on..if you solve this math problem - I'll kiss you"

he chuckled.. seems to work?