An inker is a member of a comic book or comic strip's creative team. The inker is mostly in control of a drawing's line width, which can significantly change the appearance of a penciller's work, especially with regards to visual depth and mood.
History of the term
The term derives from the mostly historical production process in which the primary artist, or penciller, handed the inker a drawing that was completed in pencils. The inker would then add ink to the pencils. If the piece were monochromatic, it would then be finished, save for lettering. If it were in colour, it would then move on to the colourist, prior to being lettered.
In the 21st century, comics are more rarely physically inked, but instead "inked" via Photoshop or other graphics manipulation programmes.
Inking was not a separately-credited talent in comics for many decades. Indeed, many pencillers inked their own work. However, as inking techniques became more sophisticated, some artists specialised in inking, and the credit became more common. In the 21st century, the inker is almost always credited, even when the work is inked by the same person who pencilled it.
In Doctor Who comics
Prior to Perkins' breakthrough, inkers weren't credited for two main reasons. First, Polystyle's TV Comic and Countdown comics — as well as the comics in Doctor Who annuals, were usually painted, rather than drawn. Thus, there literally were no inkers about the first fifteen years of Doctor Who comic history.
This tradition continued during the first year of the Sixth Doctor's era, until finally John Ridgway handed over inking chores on Profit of Doom to Perkins. While there were still occasional strips inked by the penciller after Profit, inking credits became much more common from that point forward.