WARNING: This article may contain significant information from recent episodes that may be considered spoilers.
The Watcher Edit
After assigning Barnaby detention, Mr. Miller reveals himself to Layla as being her Watcher, part of a shadowy organisation known as the Watchers' Council, responsible for the training of and supervision of the Slayer. He invites her for a test later that night.
Due to Imogen and Barnaby catching Layla sneaking out of the library after going to the library to do some revision, they and Robert catch Layla talking with Mr. Miller in the cemetary, and assume that the two were bonking.
Eventually, it is revealed to the other three that vampires and magic are real, that Layla is the Slayer and Mr. Miller her watcher.
Mr. Miller is revealed to be missing at the end of the first episode, with the Biology classroom a police crime scene. Layla and the group find clear signs of a struggle, with a crossbow bolt pinning the detention list to the ceiling. A later attempt at investigating Mr. Miller's house proved similarly unfruitful, with Barnaby breaking a statue and with no traces of him, though Layla managed to purloin his crossbow.
Determined to Prove a Villain Edit
This section contains significant information about recent episodes or about a character that may be considered spoilers.
By episode 2 the cast found out that he has been attacked by a group of vampires possibly during detention. At the end of episode 2, as the other three were on their way to rendezvous with Imogen at the park, he kidnaps Imogen Parker and is seen holding her captive with a group of student-vampires. Imogen was then captured and tied to a support in a disused building at the edge of the school. Layla, Robert and Barnaby moved to rescue Imogen, with Layla failing to kill Mr. Miller with a stake to the heart due to it shattering on a cuirass Miller was wearing under his coat. With his cronies heavily engaged and the building collapsing, Miller brought down the building, forcing Layla to hold up the column to stop Imogen from being crushed. Miller escaped as his cronies were finished off.
As of episode 3 he remains at large and unaccounted for.
Mr. Miller remained a thorn in the sides of the group. At one point he returned as a Biology teacher, surprising Barnaby and Imogen, as Layla and Robert had skipped class. He had fiddled with the gas supply in the Bio lab, using this as leverage against Imogen to stop her or Barnaby from attacking him, as him turning to dust might cause a gas explosion.
Imogen managed to defuse this situation, getting Barnaby to open the windows and the blinds to lower the risk of Mr. Miller burning up. Mr. Miller quickly beat a retreat after being literally backed into a corner to avoid being burned up, but managed to order five students to go to the office. He killed them, thrusting a pipe through their chests and leaving them in the girls' toilet for Imogen to find, earning her hatred and causing a desire for vengeance.
Mr. Miller would fall to the side as the group's search to heal James increased, but he did not fail to act. After the group celebrated killing Penemue, Miller, in an incredible, superhuman achievement successfully used an SA80, one of the worst assault rifles used by a modern military (which he stole from an airdrop Robert called in to) kill James and gravely wound Robert. An empowered and enraged Barnaby and Layla chased after Miller, who had made the SA80 inoperable (not a hard task).
They chased Miller to a warehouse, where he engaged them with a group of armed vampire cronies, himself armoured and wielding a broadsword. These managed to tie down mega-Barnaby and Layla's attention for a while until Robert intervened to break the deadlock with an improved firebomb. Simultaneously, Imogen managed to restore Miller's soul to his body using the heart of the Golem, with his vampire soul and human soul coexisting. Conflicted, Miller fell to his knees in regret at the actions he had done. Offering little resistance, he was beaten up by Layla before Barnaby and Layla quarreled over who would kill him. Ultimately, Layla would be the one to kill him. With that Mr. Miller died, heart full of regret, unmourned and unloved.
With his death, the Watcher's Council may have begun to realise that Layla's Watcher was compromised or no longer in action, and may have taken action as a result. Previously, Mr. Miller had managed to distract them from finding out about Layla's activities or his being turned.