Magister Prior Gilhil

Magister Prior Gilhil was a Plumber who made the rank Magister (an equivalent to a military officer's rank). He was shown to be very strict and followed the rules set by the Plumbers closely.

Alien Force
The Magister greatly resented (in Darkstar Rising) that three human adolescents were claiming to be Plumbers and was even willing to throw Ben Tennyson, Gwen Tennyson and Kevin Levin into the Null Void forever for such a crime, and he only spared them from the Null Void because they, as Gwen pointed out, doesn't claim to be Plumbers but simply protect people from alien attacks (and especially in Ben case, since his Omnitrix already gives him a sort of Plumber license). However, he forces Kevin to stop fighting with aliens, regardless of self or the others defense, due to his criminal files.

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 already. It is possible that Gilhil 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.

Ultimate Alien
In 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 Gilhil from behind. This act killed the Magister Plumber. Aggregor appeared later wearing his Plumber suit.

Ben 10: Alien Force

 * Darkstar Rising (first appearance)

Ben 10: Ultimate Alien

 * Escape From Aggregor (first re-appearance; flashback; death)

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.
 * Gilhil only appeared alive in the episode Darkstar Rising. He was then killed by Aggregor in Escape From Aggregor in the flashback.
 * It is confirmed by Dwayne that Gilhil was Max Tennyson's boss.
 * Because of his sudden death it is currently unknown of what became of his body.