Helios (The Infinity Doctors)

Helios was an inhabitant of the Librarinth on the Needle at the end of the universe. He was the only of the residents there with the ability to remember the past as well as the future, though he only had snatches of memory of both. As a result, he alone remembered pieces of his life before coming to the Needle to continue his father's work, and he alone knew the route to the door leading to their God.

When the Doctor arrived on the Needle, Helios was an old man with a neat white beard carrying a big wooden staff. Willhuff suggested that Helios used to be Merlin. When the Doctor arrived at the door to God, Helios was waiting for him, and they engaged in a short argument about whether the Doctor should open the door. During the conversation, the Doctor realised that he recognised Helios, and Helios admitted that he knew the Doctor, though he wasn't exactly sure of the details of their past. After warning the Doctor that, if he opened the door, Gallifrey's fate in the hands of the enemy would be unavoidable, Helios reluctantly let the Doctor pass. "I have lost everything now, everything but my faith. I tried. I saw the future, I knew that I was destined to fail to convince you. But I tried."

- Helios

When the Magistrate and Larna arrived in the Librarinth, Helios tried to run away from them, but the Magistrate hunted him down and stuffed a Z-cap in his mouth. (PROSE: The Infinity Doctors)

Behind the scenes
"The idea was that there would be a war criminal type in both our books, and depending on which one you read first, you'd get a completely different appreciation of the situation. If you read the book in which he was an old librarian who just wanted to be left alone first, you'd think the Doctor was a bully. If you read the book where you saw him in his prime, massacring innocents, you'd want the Doctor to murder him in cold blood."
 * Author Lance Parkin suggested that the character of Helios might have been originally conceived as a link between The Infinity Doctors and its never-made tie-in novel Mentor.

- Lance Parkin