Which character is described as Tim Canterbury's vindictive deskmate and enemy?

Gareth Keenan is described as Tim Canterbury's vindictive deskmate and enemy.

Played by Mackenzie Crook, Gareth is the self-important team leader at Wernham Hogg. He treats his unofficial authority with extreme seriousness, while Tim sees the role as little more than extra responsibility for the same pay. Their tense desk arrangement fuels many of the show's recurring jokes.

Tim regularly pranks Gareth by hiding his computer, putting his stapler in jelly, gluing his telephone receiver down and misleading him with elaborate military talk. Gareth's rigid behaviour and lack of humour make him an ideal target, although he sometimes appears to be friendly with Tim before another prank resumes the rivalry.

Gareth is also a former Territorial Army member and repeatedly boasts about military knowledge. His job title is another frequent source of confusion: he calls himself “Assistant Regional Manager,” but the more accurate phrasing is “Assistant to the Regional Manager.” After David Brent leaves, Gareth eventually becomes regional manager, without gaining much respect from the staff.