Ian Marter (in-universe)

Ian Marter was a friend of the Doctor. The Fourth Doctor acknowledged him as "a friend and a good egg." (PROSE: Scratchman)

Behind the scenes
Ian Marter is mentioned only in the Acknowledgements for the novelisation of Scratchman. This of course acknowledges the real-life Ian Marter, who played Harry Sullivan in the TV series, and who planned out the original story for Scratchman together with Tom Baker. The whole section is only a sentence long, reading: "I would like to mention Ian Marter as a friend and a good egg."

However, the novel has a very metafictional presentation, written as it is in first person from the Fourth Doctor's perspective. The audiobook, read by Tom Baker in character as the Doctor, also has a few additional lines and passages not present in the text, furthering the illusion that the Doctor himself is the book's author. It is still in this style that the book presents three additional chapters which follow the novel's Epilogue: "PS From the Doctor", "Acknowledgements", and "A Note from Sarah Jane Smith". The first and third of these chapters are still presented in-universe. The Acknowledgments, being nestled in between the two, are therefore read before there is any clear break in the in-universe presentation.

The metafictional elements and structure therefore point to the Ian Marter mentioned in the Acknowledgments as a fictional individual native to the Doctor Who universe, who met and befriended the Doctor. Such a thing is not without precedent, with Tom Baker notably among others also having been established as having an in-universe counterpart.