Could You Believe Your Eyes
Harry had an accident when he was very young (4-6?) and he gained an OC SI another soul and with it a voice in his head. This voice educates him on possibilities of magic. He had accidentally got better marks than Dudley, but when Dudley was about to tell his parents, Harry created an illusion that Dudley had knocked over and shattered a very elaborate glass vase, so that the Dursleys fussed over their son and forgot about Harry. He later just threw the vase out.
When Harry comes to Hogwarts, he's quieter and a bit more studious, so Ron isn't his best friend. Instead, during the flying lesson he doesn't go up in the air but lets Malfoy drop the remembrall, hides it with his magic until he can put it in his pocket and creates an illusion that it shattered. Malfoy gets in trouble for flying and breaking another student's possession. Harry also encourages Neville to believe in himself and thus grow into his power.
Stuff happens, it's the fight in the ministry and Sirius has fallen through the Veil. Harry fights Voldemort in front of the ministry workers. They circle so that Voldemort has his back to the lifts, while trading barbs. Voldemort brags that Harry will never be powerful enough to kill him, but Harry pauses and says "You're already dead", and Voldemort collapses.
Standing in front of the lifts, Sirius lowers his wand.
(Harry hadn't used his illusions for a while, but he used one to make it look like Sirius died, so that the death eaters could be caught unaware)