Magister Prior Gilhil

Magister Prior Gilhil was a Plumber who made the rank Magister (an equivalent to a military officer's rank, possibly Colonel or Lieutenant Colonel). He was shown to be very strict and followed 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.

Gilhil 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 Gilhil 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 the same episode wearing a plumber suit.

It has been confirmed by Dwayne McDuffie that Magister Prior Gilhil was actually killed by Aggregor's attack. This would explain how he got the plumber suit and Plumber ship.

Trivia

 * Gilhil is the second known Magister Ranked Plumber that has been killed in the line of duty. The first was Magister Labrid
 * Magister Prior Gilhil has over 300 planets under his watch as mentioned in Darkstar Rising.
 * In Darkstar Rising Gilhil only appeared in the episode alive outside of the flashback in Escape from Aggregor.
 * It is confirmed by Dwayne that Gilhil is Max Tennysons boss.