|First appearance||Vol.5/Chapter 24/Episode 6|
Gil acts as a mascot for Licht Jekylland Todoroki to promote the latter's piano play, so he mostly wears a blue whale costume with grey hair on the left side of the costume's head. During Lawless' times as Ophelia's Servamp, he is shown to be wearing a medieval suit of armor. Güldenstern's true appearance is an old man with a thick beard.
Guildenstern is dedicated in his job both as Lawless' subclass and a mascot for Licht. Even though he is annoyed by Lawless' childishness, referring him as a "shitty brat", he still considers Lawless his boss and dutifully accompanies him for years. Like Crantz who acts as a "babysitter" for Licht, Gil also acts as one for his master, punishing him if he misbehaves.
While the exact time Guildenstern became Lawless' subclass is unknown, it's certain that Gil has served as Lawless' subclass for long times since he was present when Lawless served Ophelia, a princess of an unknown country that existed hundreds of years ago. During this time, Gil is shown approached by Lawless who was looking for Ophelia and asked did he saw her. Gil pointed the way Ophelia went to, earning him a scold from Lawless for letting her go, to which Gil retorted in his mind that Lawless should just shut up.
Gil first appears together with Crantz, stopping Licht and Lawless' fight by bumping their heads to each other. He is then introduced by Crantz to Mahiru as Licht's mascot and Lawless' subclass as he was tending the injured Licht. The next day, he and Crantz visits Tetsu's hot spring lodge after hearing of Lawless' disappearance and is surprised to hear that his Servamp and Eve have been kidnapped by Tsubaki. He and Crantz, together with Misono, Tetsu, Lily, and Hugh, arranges a plan to save the Greed Pair.
As planned, he and four other subclasses confronted Tsubaki at the promised place to exchange hostages. He tells Tsubaki that even though Lawless is a brat, he is still their boss and punishing the Servamp has been their long-standing job, not Tsubaki's, demanding the Melancholy Servamp to return their master. Tsubaki asks where his subclass is, and Gil answers that Lila is inside one of their costumes before he attacks Tsubaki. This is in fact ruse to delay Tsubaki while Misono and the others rescue the Greed Pair. He and the other subclasses are eventually defeated by Tsubaki, and he is taken by C3 to their HQ to tend his injuries. There, he makes contact with Lawless and Licht through the monitor in their rooms, revealing his real face to Licht for the first time.
- "Even though he is a shitty brat, a boss is still a boss. It's our long-standing job to punish that brat."
- Just like Ophelia and Rosen Crantz, Guildenstern is also the name of a character in Shakespeare's Hamlet
- Guildenstern is the only subclass known from Lawless