You are number 8 of the 8 Hargreeves children. You have the power to mimic anyone's powers and use them to your advantage. Luther is number 1, Diego is 2, Allison is 3, Klaus is 4, Five is 5, Ben is 6, and Vanya is 7. You died on a mission with Luther, Diego, Klaus, Ben, and Allison when you were children. Your siblings were brainwashed by your guys' father to believe that ypur death was their fault for not working as a team. But what really happened is that your guys' adopted father, Sir Reginald Hargreeves, murdered you due to a slip up. Yet no one remembers. Your death was 16 years ago.
Klaus, number 4, had the ability to communicate with the dead. So while Allison, Luther, Diego, Ben, Five, and Vanya mourned your death, Klaus could still speak to you, even though he's still alive. Like you never died. Your guys' siblings and adopted parents never believed Klaus when he told them that he could speak to you, so he gave up on trying to tell them.
You had to watch Klaus go through being addicted to drugs and alcohol and going to rehab and being relased from it over and over again through the years. Then the world was in danger of ending, and your siblings banded together to try and save the world.
While Klaus and Diego, your favorite brother, were together, trying to get back to Allison, Ben, Vanya, Five, and Luther, Klaus allowed you to posses his body so you can speak to your siblings. When you found Diego, he thought he was speaking to Klaus since you're in his body. You were excited to speak to your brother again, so as Diego was telling you, who he thought was Klaus, the plan, you expressed your joy of speaking to him again.
"Wow, you really are high," Diego chuckled, not knowing you possessed Klaus' body and not believing it until you proved it.