In The First Time, Blaine Anderson takes Sebastian Smythe to Lima Bean for a "Dalton to Dalton" talk. This is when Sebastian first admits his desire to be with Blaine despite Blaine already having a boyfriend. Kurt walks in just after the conversation and immediately feels a dislike for Sebastian.
In Hold On To Sixteen Kurt and Blaine are venting about the sorrow in New Directions. Sebastian makes another appearance here and, when Blaine leaves for a minute, throws insults at Kurt which are returned. He promises Kurt that he will not only be leading the best Glee Club around but also have Blaine as his boyfriend.