Joel McTiennan is an Interpol agent who was assigned to investigate the Brown family's disappearance with Agent Maria Medina.

Because of their different builds, McTiennan and Medina are affectionately referred to as "Big Guy and Small Fry" by their colleagues.

Personality

He is a man of few words and is intimidating with his strong frame and no-nonsense attitude. He thinks that Agent Medina's theories are off-the-wall and that the case is just a dead-end and they will return empty-handed.

Chronology

Comics

Maria and Joel

Agent Joel McTiennan, along with Agent Maria Medina, was first introduced in Issue 010: A Bridge Between Two Worlds investigating Elyon Brown's disappearance. They were looking around the Brown's home for clues to the family's disappearance alongside Tom Lair and McTeinnan found a photo of Elyon and the W.I.T.C.H. girls taken at the Halloween party (in Issue 001: Halloween) and recognized one of the girls as Irma Lair, Tom Lair's daughter.

This led to Will, Taranee, Cornelia and Hay Lin visiting the Lair house, seeming under the pretense of joining the family for dinner, but in reality, it was so that both agents can interview all five girls about Elyon.

Medina thought the W.I.T.C.H. girls were hiding something that would be helpful to their case, so as a result McTiennan and Medina tried to keep a close eye on the girls. They held a stakeout at Cornelia's home. Will, trying to smuggle Cornelia away, brought a disguised Vathek and an Astral Drop of Herbert Olsen, Matt Olsen's grandfather, who barely knew how to drive. Using this as an advantage, the Astral Drop asked both agents questions about driving which acted as a diversion while Will and Cornelia snook into the van where Vathek was hiding.

As Matt's "grandfather" drove away, the van's back doors flew open, revealing Cornelia and Vathek for a moment. Medina, seeing this, ordered McTiennan to follow them. Their car wouldn't start, and McTiennan briskly went to have a look at the engine. He found plants twisted around the engine, thanks to Cornelia, who helped a small plant directly under the car to grow.

In Issue 011: The Crown of Light, Medina and McTiennan are seen in the Brown Family's basement, looking at a wall which does not appear on the house's blueprints. (This was put up by during a magical fight in Issue 002: The Twelve Portals.) McTiennan tried to break down the additional wall, initially with a large hammer, then suggested using dynamite. Agent Medina said he should stop so that the house is not destroyed.

Medina and McTiennan are next seen in Issue 025: Water Shadows, where they talk about the strange incidents that all five W.I.T.C.H. girls are all seen around, alongside Raphael Sylla, also on the police force and investigating the girls. They are also seen discussing the same issue with Nora Frieder, also on the police force and who asked Medina and McTiennan to spy Sylla and his investigation.

Later in the same issue, Medina, McTiennan and Scylla are all seen working together to stop Riddle. The confrontation, guns were drawn and pointed at different parties in a standoff. This was resolved when The Oracle pulled everyone there to Kandrakar, where he used a memory spell on everyone there to remove their memories of W.I.T.C.H..

TV series

In the TV series, Maria Medina and Joel McTiennan are hired after Alchemy raises questions about Elyon's absence and are fooled by Miranda and Sandpit into thinking that the Guardians were responsible for Elyon's disappearance in C is for Changes. The two detectives nearly arrest W.I.T.C.H., but but Elyon with her parents arrive with Caleb and Blunk (glamoured as a puppy) at airport to end Maria and Joel's suspicions staging that the family was on a trip.

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