Magister Prior Gilhil

Magister Prior Ghilhil is a Plumber who made the rank Magister(an equivalent to a military officer's rank, possibly Colonel or Lieutenant Colonel). He is shown to be very strict and follows the rules set by the Plumbers closely. The Magister greatly resented(in the episode Darkstar Rising) that three human adolescents were claiming to be Plumbers and was even willing to throw Ben, Gwen and Kevin into the Null Void forever for such a crime.

Ghilhil was quick to side with a Highbreed's claim of being attacked over Ben and his team, simply because the trio was on thin ice with him before. It is possible that Ghihil holds grudges due to his lack of sympathy when seeing Ben get in trouble a second time, but he managed to keep his head cool enough to ask the Highbreed to come with him for questioning. The Magister later shows a softer side when he allows the trio to keep their Plumber badges after realizing that Darkstar was behind the whole incident.

Ben 10: Ultimate Alien
In the episode Escape From Aggregor Magister Ghilhil answered a distress call from Galapagus, aboard Aggregor's ship, only to be told on arrival that it was a computer error. After demanding a search of the ship, he soon locates Aggregor's prisoners. The Magister's obsession with the rules backfires on him while he announced Aggregor's code violation and listed his crimes. Unknowingly, this gave the Osmosian just enough time to attack Ghihil from behind. Aggregor appears later, in another episode, wearing Ghilhil's uniform. It has been confirmed by Dwayne McDuffie that Magister Prior Ghilhil was killed on screen in Escape From Aggregor.