Marquez was an android and a Dalek agent.
When the Eighth Doctor first met Marquez, he was working for the scientist Crivello in the 51st century. After Crivello's Cauldron was finally primed and departed to create a new sun, Marquez killed Crivello since he was no longer needed. (COMIC: The Keep)
Two centuries later, the Doctor encountered Marquez on Icarus Falling. The android revealed it had been working for the Daleks. The android had been helping them so they could use the Cauldron to open a time rift, and destroy Daleks from parallel universes, whose existence they could not tolerate.
However, the rift allowed two-way travel, and other universe Daleks came through to attack the ones from this universe first. As they destroyed each other, Marquez tried to gain access to a box of Time Lord secrets paid to the Threshold to cause the Daleks' destruction. Sister Chastity killed him so she could keep the box for herself. (COMIC: Fire and Brimstone)