Summary: Kurt didn't really exist, he was only there to help the others through.
When Elizabeth got sick Burt felt something in him break, not enough for most people to notice but enough, and Kurt Elizabeth Hummel was born.
Kurt was eight when his Mother died and no one could tell Burt any different, Elizabeth had always loved fashion and despite being a boy so did Kurt, he like the expensive things, designer. And so Burt gave the boy a reason, the child was gay and they had known it since he was three.
When she was nine, Brittney moved in across the street from the Hummel's, she heard about the strange man who had a child that didn't exist. And he didn't to her for a while, at least not until she had to start a new school all alone, so she introduced all of her new friends to Kurt Hummel, her neighbor and her best friend.
It was a simple thing for a parent not to notice, but Burt came to love Brittney for the fact that she would come over to play with Kurt, the young boy had been lonely after his mother's death.
It was all rather simple for many of the Gleeks to accept the member of the club, they needed another member to make the competition, and Brittney introduced them to Kurt.
But the adults seemed not to notice when Kurt had a problem, because they couldn't see Kurt like the children did, Mister Shuester couldn't pick Kurt for a solo and Mercedes bonded with the boy over that. Finn was caught in a love triangle that was so weird that he added Kurt, so that there was someone else to blame and Rachel picked up on it and they came to an agreement about themselves.
And slowly everything became easier, it was easier to include eachother in the delusion that Burt Hummel had begun all those years ago, it was easier to pretend that Kurt was really there, and when Rachel needed someone to be with her and Finn when they opened their letters Kurt was there and Kurt didn't get into NUADA like Finn didn't get into his school but she got into hers and that was okay because Kurt Hummel didn't exist.
After a while it became easier to let Kurt go, as their lives progressed, as their dreams came true, Kurt became only memory, an image in Sam's Comic book, a boy smiling with the Gleeks, dressed in his Designer clothing, hair styled perfectly and if in some of the panels the outlines of wings happened to spout from his back, then that was that, and only a few people ever noticed it and those people were the ones that needed a Kurt.
For the longest time Brittney would write to Burt Hummel, she would address her letters to Kurt Hummel, but she knew that Burt was the one that read them, a few times she even asked why Kurt did not reply to her letters, until finally Burt admitted to himself that Kurt Elizabeth Hummel had never existed, that he was a hope that he had had before Elizabeth had died, a hope that had never had the chance to come true.
It was only after that that Brittney stopped writing, and she never did again, because Kurt Hummel didn't exist except in the minds of the members of the New Directions when they had needed him.
Right, so I have no idea where this came from, but it made me write it.
It has been a while since I watched the series, and I haven't watched the fourth season yet so that should explain the gaps.
Prompted by this thought: He didn't really exist, he was only there to help them.