Matthew Finnegan

Matthew Finnegan was a human who worked for a film studio in 1920s Hollywood.He travelled with the Tenth Doctor as his companion for a time. (IDW: Silver Scream, Fugitive) He later sacrificed himself to save the Doctor and his friends, becoming the Tef'Aree, a legendary pan dimensional being, which later turned him into an evil manipulator, which shattered the Doctor's faith in him forever.

Travels with the Doctor
During his first adventure aboard the Doctor's TARDIS, he found himself overshadowed by fellow companion Emily Winter, which seemed to displease him. He was easily manipulated by the Advocate into betraying the Doctor. The reason he gave was that he was protecting Emily and he was terrified of dying like Adric (which he learned about from what was supposedly Vislor Turlough's journal.) When the Doctor discovered this, he was furious with Matthew and they parted company. Matthew told the Doctor he trusted the Advocate more than him, so he would travel with her for a while. The Doctor was appalled by Matthew's behaviour, but still threatened the Advocate should she harm Matthew. (IDW: Tesseract)

Rebirth
Matthew spent the next two years of his life beside the Advocate, without realising he was a mere slave. Emily found him and slapped him for his betrayal and told him that Turlough had confirmed that he never wrote a diary. Matthew then felt a great amount of guilt for what he had done to the Doctor and Emily, and joined forces against the Advocate. When Matthew sacrificed himself to save a race from annihilation, killing the Advocate in the process, their energies combined, and Matthew was filled with fifth-dimensional energy. He was reborn as the first and only fifth-dimensional being, the Tef'Aree, despite the death of his original human body. He lost linear time and became present at all the Universe's history.

To ensure his rebirth as the Tef'Aree, Matthew manipulated events leading to his creation. He allowed the Advocate to reform from spacedust, diverted an Anochian ship to 16th century Greenwich, saved Emily Winter, gave the humans from Colony One terraforming and teleport technology and showed Leo Miller how to travel to 1920s Hollywood. He appeared after his 'death' to the Doctor to confirm all this. He was chastised for manipulating everyone, including his past self, just to ensure his own creation. The Doctor turned away from Matthew forever in disgust. The Tef'Aree reminded the Doctor that he was Matthew, despite his protests to the contrary and left, saying they would meet again. (IDW: Final Sacrifice)