Death's Head

Death's Head was a metallic life-form who worked as a "freelance peacekeeping agent" (effectively, a mercenary, bounty hunter and assassin, though he hated being called any of those things) in a number of dimensions and time zones. Death's Head described himself as a mechanoid, a designation not related to the Mechanoids, the enemies of the Daleks.

Biography
Death's Head did not take pleasure in killing, merely in being professional and financially astute, and possessed a rather pedantic personality. He was a habitual traveller between alternate realities, spending most of his career in universes other than the main one. At the time that he first encountered the Seventh Doctor, Death's Head had been enlarged to nine metres in height.

While travelling through the Time Vortex, Death's Head collided with the Doctor's TARDIS, forcing both to materialise. Death's Head attacked the Doctor, but was shot with a TCE and returned to his original size apparently uninjured. The Doctor eventually used the TARDIS to launch the mechanoid through time and space to the planet Earth in the year 8162. (DWM: The Crossroads of Time)

Death's Head later attempted to claim the price put on the Doctor's head by Josiah W. Dogbolter, using a prototype time machine to track him down. However, Dogbolter meant to double-cross the mechanoid, hiding a nuclear device in the time travel unit to kill both him and the Doctor. The Doctor saved Death's Head from the bomb and in return the mechanoid decided not to complete the contract on the Time Lord... this time. (DH: Time Bomb!)

On a different occasion, Death's Head, while counting money, raised his glass in acknowledgement to the Doctor at Bonjaxx's party. When a drunk Beep the Meep started a fight, Death's Head happily joined in. (DWM: Party Animals) On the way out of the party, Death's Head was attacked by Dogbolter's old servant Hob but was saved by teaming up with his own future self. The Doctor erased the memory of this encounter from Death's Head, and was revealed to have manipulated Death's Head II into being in this situation. (The Incomplete Death's Head #12)

Behind the scenes

 * Death's Head was the lead character of a comic book published by Marvel UK which had a couple of crossovers with Marvel UK's Doctor Who comic strip. He also appeared in the pages of DWM. The practical upshot is that the Doctor is a part of the wider Marvel Universe.  It's mostly a matter of logical inference — and therefore tangential — but there is one fairly direct scene.  In the Doctor's only appearance in Death's Head (1988), he drops Death's Head off on top of the Baxter Building.  In the next issue, Death's Head is involved in an adventure with the Fantastic Four.  Thus there is a direct and unmistakable link between the Doctor and Reed Richards. Some Marvel scholars have therefore assigned the Doctor to Earth-616, but other Marvel Universe alternatives are possible.
 * One of the crossovers between the DWU and Death's Head doesn't involve the Doctor at all, but rather a character named Keepsake who had originated in Doctor Who Magazine.
 * The character of Death's Head has had three major iterations. The first one — that is, the star of Death's Head (1988) — is the one that's had the greatest interaction with the Doctor. That said, there is a very tenuous connection between the Doctor and the second version of the character implied in the pages of The Incomplete Death's Head, a series that mostly just reprinted Death's Head (1988).  Death's Head 3.0 — seen in mainstream Marvel US comics — has absolutely nothing to do with the DWU.