What is the explanation of Yuki sending e-mail to Naoto?


Naoto boasts about his ability and gives 30 minutes handicap to other players.
Then surprisingly Yuki sends a mail to Naoto in which she asks Naoto to stop
fooling around.

I'm very confused at this point. I have finished the series and I haven't seen
Yuki and Naoto together even once. There is not even another sign about they
knew each other. What is the background of this intimate message? Where does
they know each other?

![enter image description here](https://i.stack.imgur.com/vCLrD.png)  
( _Episode 11_ )

