In A Very Glee Christmas, Kurt reveals to Will that he is in love with Blaine, and that Blaine is actually gay.The two perform their first duet, Baby, It's Cold Outside.
Their relationship is taken to the next step in Season Three when Blaine transfers to Mc Kinley and the New Directions, because he can't stand to be apart from the person he loves.
Kurt and Blaine share loving looks during some songs and rarely fight, only having three fights during the course of their relationship so far (The First Time, Dance with Somebody and The Break-Up).
They begin dating in Original Song, after Blaine kisses Kurt, forming the first male-male couple on Glee.
In The Purple Piano Project, Blaine transfers to Mc Kinley to be with Kurt, telling him that he can't stand being apart from the person he loves.
Blaine flirts with Kurt, and in Wonder-ful Blaine asks permission from Burt to ask Kurt to marry him.
Burt, while happy that Blaine is so in love with his son, explains to Blaine that they are too young, and that they are meant to be together so he should just relax and everything will be OK.This is the episode in which they share their first kiss, and it is assumed that they start dating very soon afterwards.In the episode New York Blaine tells Kurt that he loves him for the first time, to which Kurt replies that he loves him too.The couple break up in The Break-Up, but they started dating again in Love, Love, Love, where Kurt accepts Blaine's marriage proposal.In New New York, Blaine moves to New York to live with Kurt, but they soon realise that their relationship is stronger if they live separately, and he moves out.Then in Silly Love Songs, Kurt admits to Blaine that he has feelings for him after Blaine fails to serenade a guy he likes at "the Warblers Gap Attack." Blaine confesses that he has no idea what he is doing when it comes to romance, and the two agree to stay friends.