Father Kreiner, formerly Fitzgerald Michael "Fitz" Kreiner, was a human who travelled with the Doctor before becoming trapped on Ordifica and joining Faction Paradox. A copy of his biodata was left with the Remote on Anathema and — after several generations of remembrance — a copy of Kreiner (called Kode) rejoined the Doctor.
Original Kreiner Edit
In San Francisco, 2002, Fitz worked as a paranormal investigator for the Doctor whilst investigating Griffin the Unnaturalist. He had sex with a dark-haired version of Sam during this period. (PROSE: Unnatural History)
When the Doctor, Sam and Fitz landed on Earth in 1996, three or four distinct iterations of Fitz Kreiner were brought into existence. Fitz was placed in the Cold by the United Nations in 1996 in Geneva and removed from it in 2593 on Ordifica. Fitz decided since he was so far from the Doctor and his own time, he would become an agent of Faction Paradox. (PROSE: Interference - Book One)
On joining the Faction Paradox, Fitz and the other survivors of Ordifica were taken back in time. They were placed on Anathema by the Faction Paradox to become their troops, the Remote. Fitz joined their society without actually becoming a part of it in the literal sense. While on Anathema, he contemplated suicide. He joined the Faction and was indoctrinated into its ranks.
During his time with the Remote, Fitz hunted and killed many renegade Time Lords, including the Rani and the Master. One or more of these kills may have been of clones. (PROSE: Interference - Book One)
Father Kreiner led his troops onto the planet Dust and attempted to kill the third Doctor for his future self's abandonment. Kreiner tried to follow the Doctor by grabbing onto I.M. Foreman's Travelling Show when it assumed the form of the TARDIS, believing it to be his Doctor's ship, but was flung into the Time Vortex. He was plucked out by I.M. Foreman and trapped in her bottle universe's Time Vortex. (PROSE: Interference - Book Two)
It took over two centuries for Anathema to travel to Earth. During this time, Remote-Fitz was remembered repeatedly. By his last iteration he was called "Kode" after his nickname, "Code-Boy". Kode came to Earth selling the Cold with Compassion and Nathaniel Guest.
Some of Fitz's personality traits came through the process, so the Doctor was able to recognise him. As one of the Remote, he was open to the Doctor's TARDIS's signals, which made him more open to the Doctor's cause. Kode preferred not to remain as himself as he would always be left wondering how he compared to the "original" Fitz, so he allowed the Doctor to restore him to who he had been. And so Fitz Kreiner was re-re-remembered, using a remembrance tank and the TARDIS's memories of him. To the Doctor, who assumed the original Fitz was dead, Kode was Fitz in every way that mattered, even if he wasn't the "original." After that, "Kode" essentially ceased to exist, although the Fitz that resulted from the re-remembrance retained some vague memories. (PROSE: Interference - Book One, Interference - Book Two)
New Fitz Edit
Kode-Fitz travelled with the Doctor and Compassion to Gallifrey, where he tried to prevent Faction Paradox from invading. Father Kreiner was retrieved from the vortex, and the two Fitzes met. Kreiner initially considered Kode-Fitz a mere copy of himself. The Doctor managed to convince Kreiner that he genuinely had only abandoned Kreiner because he thought he was dead, so Kreiner tried to help him. Grandfather Paradox shot and killed him for his betrayal, leaving Kode-Fitz the last surviving Fitz. The Doctor destroyed Gallifrey, and Compassion transported Fitz to Earth to meet the Doctor in 2001. (PROSE: The Ancestor Cell)