While they're fooling around, Mai finds a picture Santa has of a girl.
She's his senpai from the club. Someone he admires. (Yeah right.)
Mai works some magic and Santa gets a sudden call and an invitation.
But Mai is a bit jealous anyhow, and hangs up the phone.
Why would she do that, he asks. Playing around about Santa is fun and all, but this was real.
Mai is crushed. Why would he say something so cruel?
And since we're speedrunning the anime tropes, as she runs off she admits that she fell in love with him.