Elite Four (FireRed)

The Elite 4 represent the highest challenge for any Pokémon Trainer. Defeat all four, and become Champion; though you'll have to defeat the existing Champion if there is one. In Randomized FireRed, the Elite 4 all had their types changed. Though they gloated about their legendary skill with their storyline type, they often used a type completely different.

Lorelei
Lorelei, originally an Ice-type Trainer, now uses the feared Ghost type. Her team consisted of two Dusclops, a Banette, and two Gengar.

Trivia
Her sprite takes inspiration from Mismagius, despite it being a Generation 4 Pokémon.

Bruno
Bruno, originally a Fighting-type Trainer, now uses Normal types. His team consists of two Pidgeotto, a Zangoose, a Raticate, and a Snorlax.

Trivia
Bruno has finally put on a shirt according to his fan-made sprite. Many rejoiced at this.

His Snorlax proved to be quite a tank, combining high defense, high health, and the attack Softboiled to heal.

Bruno shares a type with Norma (Sabrina).

Agatha
Agatha, originally a Ghost-type Trainer, now uses Water types. Her team consists of a Gorebyss, Seadra, Croconaw, Mantine, and a Milotic.

Trivia
Many on Reddit tried to avoid pictures of an old woman in a swimsuit, leading to artwork like the sprite.

Despite the troubles the Mob had against Wallace's Milotic, Agatha's was easy. It was her Gorebyss that spammed healing attacks.

Lore/Personality
Agatha's sprite closely resembles Drake's from Emerald. Leading to some suspicion the two are related.

Lance
Lance, originally a Dragon-type Trainer, now uses Poison types. His team consists of a Muk, Weepinbell, a Gloom, and two Crobats.

Trivia
Lance shares a type with Noxie (Misty).

Lore/Personality
Lance is often shown to be the person who taught Noxie (Misty) about Poison and Poison types.