Stanford Pines

As of the episode "The Last Mabelcorn".[17]) (voiced by J. K. Simmons as an adult[18][19][20] and by Christian Mardini as a child) - Stan's six-fingered long-lost identical twin brother and the mysterious person who wrote all 3 of the journals. He returns from an unknown and mysterious place through a portal underneath the Mystery Shack in "Not What He Seems" after three decades. It is revealed that he has been missing since 1982. In "The Last Mabelcorn", it shows that he was good friends with Bill Cipher and even attempted to create a teleporter to enter his realm, but when he found out Bill planned to invade the mainstream dimension, he cuts off all ties with him, even destroying the teleporter, but at the cost of a dimensional rift leaking out of it. This rift was the only way from the mainstream dimension to Bill's and it is released in "Dipper and Mabel vs. the Future." In "Weirdmageddon Part 1" he attempts to defeat Bill, but is captured, petrified and his journals are destroyed. He is later freed and restored of his humanity. After Weirdmageddon, he tells Stan that he found some remaining anomalies and offers him to go sailing around the world to find them, which Stan accepts.