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 Mechonoids, an enemy of the Daleks.

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.

Biography
Death's Head was created as a cyborg body in a realm called Styrakos by Lupex and Pyra. An unknown party stole the body, programmed it with a “killer instinct” and dumped it on Scarvix, (COMIC: The Incomplete Death's Head) where Death's Head made a living as a bounty hunter. (COMIC: Death's Head!)

After some months, the Doctor transplanted Death's Head through a warp gate to a parallel universe of massive warring robots, where he was subjected to major restructuring surgery and replacement of living metal components, resulting in him being greatly enlarged. During the final battles of the war, he was caught in the gravitational well of a collapsing planet and flung into the Time Vortex. (COMIC: The Incomplete Death's Head)

While travelling through the Time Vortex, Death's Head collided with the Doctor's TARDIS, forcing both to materialise. At first, Death's Head intended to kill the Seventh Doctor, but was shot with a Tissue Compression Eliminator and returned to his original size apparently uninjured. Wishing to return to his own time, Death's Head let the Doctor lure himself into the TARDIS, from where the Doctor sent the mechanoid through time and space to the planet Earth in the year 8162. (COMIC: The Crossroads of Time)

After an encounter with Dragon's Claws (COMIC: Watch Out – Dragon's Claws Here's Death's Head!), Death's Head was repaired by Spratt. During this he remembered his time on Scarvix. (COMIC: Death's Head Revisited) He then helped the Chain Gang attack Dragon's Claws. (COMIC: Contractual Obligations)

He later had an adventure with met up with Keepsake, a salvage dealer that had once crossed the Seventh Doctor's path, and Bahlia. (COMIC: Do Not Forsake Me Oh My Darling!)

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. (COMIC: 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. (COMIC: 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. He attempted to kill his future self, but the Doctor stopped him with a modified TCE. The Doctor erased the memory of this encounter from Death's Head and returned him to the party. (COMIC: The End... Yes?)

Death's Head II
At some point, a cyborg from the year 2020 programmed with 105 "personalities" by Doctor Necker killed Death's Head and assimilated his memories, rather than just his personality. This allowed the cyborg, now known as Death's Head II, to break free from Necker's control and continue on his adventures, where he met Tuck.

Eventually the two suddenly materialised on Maruthea, where they came across the Death's Head Interactive Archive. Death's Head II was implanted into the archive, forced to re-experience his past adventures and interact with a virtual representation of the original Death's Head. Tuck was captured by Hob and forced to watch the archive. However, Death's Head II was able to return to his body from cyberspace, rescue Tuck, and stop Hob from killing the original Death's Head. When the original tried to kill his future self, the Doctor incapacitated him with a modified TCE. He revealed he was the one who took Death's Head to the "robot universe", and that he had transported Death's Head II and Tuck to Maruthea. As Death's Head II and Tuck left, he wiped the original's memory of the encounter with his future self and returned him to the party. (COMIC: The Incomplete Death's Head)

Behind the scenes
Death's Head was a character created to be a part of the Transformers Marvel line who then crossed over into mainstream Marvel, with his own series in Marvel UK. In the middle of that transition, he crossed over with Doctor Who numerous times. Sometimes he would appear in DWM, other times DWM characters (such as Josiah W. Dogbolter or Keepsake) would appear in his series. The practical upshot is that the Doctor is a part of the wider Marvel Universe, or at least was at the time.

In most of these crossovers, the references to the Marvel universe tends to be subtle. For instance, COMIC: The Crossroads of Time ends with Head on top of a building which fans might recognise as the Baxter Building. Only with the context of COMIC: Clobberin' Time! does this become overt.

However, the stories COMIC: Party Animals and The Incomplete Death's Head make the Doctor's role on the Marvel comics universe much more overt, additionally retconning that it was the Doctor who sent Head to the Transformers universe in the first place.

Marvel reference books usually reconciled that stories like The Transformers, Doctor Who and Spider-Man all took place in different universes (the Doctor's universe was usually listed as ). However, very few if any comics featured Death's Head would reference this, instead just showing Death's Head jumping through time without any other technology. At the time, all three of the above franchises were owned by Marvel, but after each fell to other publishers the divide between their universes became more overt. This is where The Incomplete Death's Head gets its name -- from the reality that no single omnibus will ever be able to completely publish his run. This is additionally why the series never states the word "Transformer" out loud.

The character of Death's Head has had three major iterations. — 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 II and his partner Tuck were led to Maruthea by a transformed Hob, in an attempt to find his old master, Josiah W. Dogbolter. However, Death's Head 3.0 — seen in mainstream Marvel US comics — has never been featured in Doctor Who story.