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).
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.
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.
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.
Near the end of the year, Blaine and Kurt are dismayed by the idea of Kurt graduating and moving to New York while Blaine will be left behind for one more year. Kurt and Blaine's relationship is tested when Blaine convinces Kurt to move to New York in The New Rachel, by singing It's Time to him.
They maintain their long distance relationship successfully at first via skyping and texting, but with conflicting schedules, things start to slide quickly.
Eventually, Kurt asks Blaine what the problem is, resulting in Blaine telling him that he was with someone else back in Lima (presumably Eli. Kurt leaves heartbroken and ends up giving him the silent treatment.