Dorium Maldovar

Dorium Maldovar was a black marketeer in the 52nd century. He was beheaded by the Headless Monks but his severed head, which retained its life and sentience, was kept in the Seventh Transept.

Dealings
In 5145, after hearing she was looking for time travel, Dorium met with River Song in the Maldovarium bar to offer her a Vortex manipulator "fresh off the wrist of a handsome Time Agent" and wonder if she had a "pretty toy" as payment for him. River offered him a Callisto Pulse which could disarm micro-explosives from up to twenty feet, like the kind she had just put in his wine. (DW: The Pandorica Opens)

Dorium sold a Judoon's brain that contained a security protocol to a trio of Headless Monks. They paid him with a bag of sentient money. As his customers left, he questioned their motives for kidnapping "the child" and told them not to provoke the Doctor's anger and rage. (WC: Prequel (A Good Man Goes to War))

Demons Run
Dorium was one of many people who owed the Doctor a debt. He was summoned to help rescue Amy and her baby at Demon's Run. He did not serve in a combat role, but hacked into the station's computer and discovered that Melody Pond had 'Human plus Time Lord' DNA.

Dorium made a fatal mistake when he and the others were surrounded by the Headless Monks. He believed they were his friends and would not hurt him. He approached them with open arms, but they swiftly decapitated him and added him to their ranks. His headless body was subsequently killed by the Doctor's remaining allies in their fight against the monks. (DW: A Good Man Goes to War)

Post-beheading
The Doctor was told by the alien Gantok that Dorium had information about the Silence. They went to the Seventh Transept, where the heads of those whom the monks had beheaded were stored. Dorium's head was in a box, alive and able to speak. He told the Doctor that on the fields of Trenzalore, at the fall of the Eleventh, a question would be asked, a question that must never be answered. The Doctor took him aboard the TARDIS. Dorium tried to explain to the Doctor that he must die at some point.

Later, a Headless Monk was seen carrying Dorium's head to the Seventh Transept. Dorium realised it was the Doctor, but wondered how he could still be alive after the Lake Silencio incident. The Doctor had used the Teselecta to fake his death. As the Doctor left, Dorium warned him Trenzalore and the Fall of the Eleventh still awaited him. Dorium called after the Doctor the first question in the universe, which he had been running from his entire life: "Doctor Who?" (DW: The Wedding of River Song)

Personality
Dorium Maldovar was an old friend of the Doctor's despite his dealings with criminals and even the Doctor's enemies. He respected the Time Lord, to the point of almost being afraid of him which was shown when he warned Madam Kovarian that it was not a good idea to make the Doctor angry. Dorium had a tendency to complain, especially shown after he was beheaded. Although he was an associate of Kovarian, the Silence and the Headless Monks, Dorium's loyalty was to the Doctor. He helped the Time Lord rescue Amy Pond, getting himself beheaded in the process. He didn't seem too bothered by his beheading, joking with the Doctor about it and commenting that living without a body actually wasn't so bad. He could keep himself entertained with a chip he had installed in his skull. (DW: A Good Man Goes to War, The Wedding of River Song)

Behind the scenes
The Brilliant Book 2012, a non-canonical REF book, puts forward these ideas;
 * Dorium would later supply Madame Kovarian with unknown infomation.
 * Colonel Manton reported to the Papal Mainframe that Dorium's head was not found after the battle.