Disclaimer: I don't own His Dark Materials, or Harry Potter.
Harry always knew he was different. There was, of course, the fact that freaky, almost like magic, things would happen if he was involved. But then there also Iritality. Who else had a constant companion, one who couldn't go very far, who was able to change shapes? So, of course, when Hagrid told him he was a wizard, that there was a whole world full of people who could do magic, Harry naturally assumed that all wizards and witches had a companion like Iritality.
When Harry asked Hagrid about Iritality, and asked where his companion was, Hagrid just looked at him strangely and said, "I used to have an imaginary friend, when I was your age. Don't worry, you'll grow out of it." Harry never brought up the subject again.
As Harry was fighting, many years later, against the Death Eaters who were intent on entering Hogwarts, a curse that was meant for him missed. At least, to everyone who was watching, it looked like the curse missed. No one else could see Iritality. But the curse hit, seeking out Harry's center in the leopard who always fought faithfully by his side.
Harry let out a scream of terror, of anguish, of sorrow. Iritality fell, her form flickering, her once full of life eyes dull. Blood was soaking her beautiful golden coat, making it look matted, rusty.
"Still beautiful." Harry whispered, tears falling down his face. "Still so beautiful, my leopard."
Iritality let out a growl. "We need to finish this. We need to stop this. If we die, everyone else will follow. We need to finish. For Hermione, for Ron, for Ginny, for Neville, for Luna. For anyone who fought with us, supported us, died for us. We need to finish it."
Harry nodded and stood up. He let the magic he had stored up inside go, let the chains he had wrapped around his core fall, let the accidental magic escape with only one thought. Kill the Death Eaters, protect the supporters.
A flow of bottled up magic flew out. Bottled up from days of staying in the cupboard. Bottled up from seven years of pretending he was normal. Bottled up, and then released.
And in the center of the show, a boy of seventeen fell to his knees and drew his last breath. And Iritality, the leopard no one else saw, appeared in her last moments of life.