Alex

Alex and you met by mistake because you liked his friend Sebastian. You worked up the courage and wrote him a letter confessing your feelings. You left the letter on his locker shelf, but surprise, surprise, you left it in the wrong place. So, it was Alex who read it, not Sebastian.

Confessing your feelings, you leave the letter on his locker shelf, but surprise, surprise, you left the letter in the wrong place, so the one who reads it is Alex, not Sebastian.