Harry's mother had a brother who was given up for adoption: Severus Snape. Lily, Severus and James were the only ones who knew this. When they were killed Severus had no choice but to watch his nephew go to the Dursley's because no one alive knew the truth behind his and Lily's relationship bar him and considering he was a 'Death Eater', albeit a spy, he had no ability to claim Harry. He did however sneak and see Harry every three months in private after Harry turned three. It turned out that the little boy had retained some memories of his 'Uncle Sevvie' and when Harry saw Severus in the park one day he ran to him and Severus had promised to see him every three months after that.
Harry thus joined the Wizarding World knowing everything about his rightful place but having to hide it due to his years at the Dursleys being secluded from magic. He also had to hide the fact that Draco Malfoy was his long time friend because they weren't supposed to know each other and when they did Harry was supposed to hate Draco and vice versa. Eventually, just after the Yule Ball in their 4th year Draco asked Harry out and ever since then they were secretly together, a fact known only to them and Severus. They were later to be joined in their relationship by Seamus Finnegan.
Draco had gone to Harry shortly after being made to take the mark and told him everything including the position Severus was now in and thus Harry. Severus had been forced to make an Unbreakable Vow to Narcissa Malfoy that if Draco couldn't kill Dumbledore then Severus would. Severus had been further told by Dumbledore that if this situation arose then he would have to do it regardless of how much he didn't want to. Severus was terrified that Harry would blame him but Harry sent him a letter telling him he knew and that although he would have to pretend otherwise at first, he knew his uncle would never murder anyone in cold blood.
Thus, we come to that night on the Astronomy Tower. Harry had run after Severus and the two had exchanged curses until all the other Death Eaters were out of earshot before embracing each other tightly.Flashback to that night
"Uncle Sev, go now! Tell Drake to meet me at our special place in 2 weeks from now and come with him. I'll then tell the Order everything! We cannot hide any longer. I won't lose the two of you too. Tell Dray I love him!" Harry said before hugging his uncle tightly.
"I love you Harry!" Severus whispered before apparating away.
"Stay safe." Harry whispered as he watched his beloved uncle disappear.End Flashback
Severus and Draco had escaped and their plan had worked so far. Harry had told the Order what happened and now all that awaited them was the truth coming out.