Joker is not only the leader of the Phantom Thieves, but he also has the "Wild Card" ability. This means that he can use multiple Personas at once, allowing him to have a ton of strengths. A jack of all trades, if you will.
In the Pokemon world, that'd be more of a Champion than a single-type Gym Leader, but for the sake of this list's theme, Joker would be more of a Dark-type user.
The reasoning for this is that he's referred to as a Trickster that can use multiple elements, but his first Persona is the legendary thief, Arsene. If being a cunning thief doesn't scream Dark-type, I don't know what does.
With all that in mind, here's how his team would look: