Dennis is a fictional character portrayed by Julian Barratt who first appears in the Series Two episode Nanageddon, then later reappears in Series Three in Eels, The Strange Tale of the Crack Fox, and Party.
He's Head Shaman, leader (and possibly King) of the Board of Shaman, the governing body who oversees the use of magic.
In appearance he is tall, with a shaven head and a silver moustache. He has pale skin decorated with simple, linear tattoos in dark green or blue. Most notable perhaps are his piercing white eyes, and his costume which usually consists of a full-length cape of green ostrich feathers and/or grasses, with a high collar of peacock feathers.
As a leader Dennis is benign and prone to being lenient with Naboo when Naboo gets into trouble (usually because of Vince and Howard ) as he seems fond of the younger wizard, but this has caused the other shamans to accuse him of being too soft. They also take delight in reminding him that he has a low capacity for drugs and alcohol and even caffeine: after three espressos he was 'found in the corner, trying to peel himself'.
In Series Three he marries 'an extreme sports calendar model'. We don't see the wedding but we do see the shamans on their way to his stag celebration. Even when married he admits to having a weakness for 'the chiquitas' which leads his jealous wife to flirt with Vince in revenge. As Dennis is also very jealous by nature and quick to use his sword, he mistakenly beheads Howard's old jazz buddy Lester Corncrake in a passionate fury. When he later spies Vince and Howard kissing on the roof, Dennis admits he has to go away and rethink a few 'basic principles'