In Generations I, III, and VII, she is an elderly woman who is the third and oldest member of the IndigoElite Four.
In the games
Agatha is a member of the Elite Four, and the oldest one to date. As such, she serves as an inspiration for senior Pokémon Trainers all around the region. She uses Ghost-type Pokémon, and other Pokémon with horrifying appearances.
Agatha and Professor Oak were once friendly rivals in their youth. However, their relationship has grown bitter and strained over the years because Agatha believes he was too weak to continue competing.
She has a very short temper.
By the events of the Generation II and Generation IV games, Agatha is no longer a member of the Elite Four, and Bruno has assumed her position. Her absence is never explained.
"I am Agatha of the Elite Four! Oak's taken a lot of interest in you, child! That old duff was once tough and handsome! That was decades ago! Now he just wants to fiddle with his Pokédex! He's wrong! Pokémon are for fighting! <player>! I'll show you how a real trainer fights!"
"I am Agatha of the Elite Four. I hear Oak's taken a lot of interest in you, child./You're the child that Oak's taken under his wing, aren't you? That old duff was once tough and handsome. But that was decades ago. He's a shadow of his former self. Now he just wants to fiddle with his Pokédex. He's wrong. Pokémon are for battling! <player>! I'll show you how a real Trainer battles!"
"Oh, my! You're something special, child!"
After being defeated
"You win! I see what the old duff sees in you now. I've nothing else to say. Run along now, child!"
"I am Agatha of the Elite Four. I hear Oak's taken a lot of interest in you, child. That old duff was once tough and handsome. But that was decades ago. He's a shadow of his former self. Now he just wants to fiddle with his Pokédex. He's a fool. Pokémon are for battling! <Player>! I'll show you how a real Trainer battles!"
"Oh my! You're something special, child!"
After being defeated
"You win! I see what the old duff sees in you now. I have nothing else to say. Run along now, child!"
Ash then challenged Agatha to a Gymbattle, hoping to demonstrate his strength to Scott. Despite being in the Top 8 of the recent Ever Grande Conference and putting up a good fight against Agatha, Ash lost to her Gengar. Despite that, both Agatha and Scott were impressed by Ash's technique, and Agatha revealed to Ash that she is a member of the Elite Four. She and Scott both recommended Ash to challenge the Battle Frontier and the Frontier Brains.
Team Rocket soon arrived on the scene, hoping to kidnap Pikachu, but Ash worked together with Agatha and her Golbat to send the trio blasting off. She then wished Ash the best for his upcoming Battle Frontier challenge and said that she would be rooting for him.
Agatha is confident in battle and always looks forward to challenging a strong Trainer. In her match against Ash, she bade her time to observe Pikachu's movements before having her Gengar attack. Over time, she has developed an array of strategies to counter an opponent, such as when she used her Golbat's Supersonic to reflect Cacnea's Pin Missile rather than confusing it.
Agatha briefly appeared in I Choose You!, which is set in a timeline separate from the main series. She appeared in Ash's nightmare, in which he lived in a world where Pokémon never existed and attended school like a normal child. In this nightmare, she was seen teaching Ash's class.
Agatha appeared in The Challenger, where she was first seen talking about Blue, the first Trainer in a long time to collect eight GymBadges, with the rest of the KantoElite Four. Later, she was the third Elite Four member challenged by Blue. After her Arbok was defeated, Agatha admitted to Blue that she had expected him, as the grandson of Professor Oak, to focus on completing the Pokédex, but Blue responded that, for him, filling the Pokédex was just a tool to become stronger. Blue managed to defeat Agatha and her Gengar off-screen, allowing him to move on to Lance's room.
Arbok is Agatha's first known Pokémon. It was seen battling Blue's Rhydon, but eventually lost.
Gengar is Agatha's second known Pokémon. It was the last Pokémon used by Agatha in her battle against Blue, first being sent out against his Rhydon and eventually losing off-screen. Agatha keeps Gengar's Poké Ball embedded on her walking stick.
When Agatha and Oak were younger, Oak defeated her in the finals for the Pokémon Championship and Agatha met him after the battle and asked why he quit their "research group". He explained that he wanted to create the Pokédex, which she thought was a stupid idea that would never be complete, and a battle ensued. Oak barely won after a battle that lasted several hours, with Agatha swearing a personal vendetta over him and anyone who associates with him.
Agatha was one of the main villains in this chapter; she was one of the Elite Four who tried to destroy most of humanity from their base on Cerise Island.
She was first seen trying to take away Super Nerd Miles after he was defeated by Yellow and the Good Gym Leaders of Kanto. She was then seen in a flashback during the events of the previous chapter where she battled Blue and attempted to kill him due to his relation to Professor Oak by having her Gengar use Dream Eater to suck away his energy. He managed to escape though and a Haunter she stole from a Trainer named Ivan was also taken back. She then had her Haunter lead an army of Gastly to attack Cerulean City to delay Misty.
She fought Blue and Koga, revealing an Arbok with the ability to change the pattern on its belly and use different corresponding abilities. She decided to divide Koga and Blue using her Arbok's Rock Slide and she had her Golbat attack Blue. After battling Koga's Arbok for a while Koga started throwing shuriken balls out of nowhere, which Agatha believed was a pitiful attempt to harm her, but was really an attempt to heal Blue's paralysis that Agatha caused using her Haunter's Lick. After that Blue rejoined the battle and Koga used his Golbat's Leech Life to collect his own blood and smeared it on Arbok's pattern, preventing it from changing anymore. While Agatha was presumably beaten, a second Arbok of hers attacked an unknowing Koga from behind, only to be stopped by Blue before it could land a Poison Sting attack. Agatha was subsequently defeated and seemingly vanished after her defeat, but not before she had her Gengar masquerade as Koga and Blue's shadow to slowly drain their energy, which was defeated as well. She then revealed that she was the one controlling the Pokémon armies of her peers by using special wrist bands to control them and she used these same bands to force Bruno to join the Elite Four as well.
Agatha used the mind-controlling powers of her Ghost-type Pokémon to control Bruno against his will. It is shown that she was the one who also recruited Lorelei by healing her Seel and then figuring out they have a common distaste of humans.
This listing is of Agatha's Pokémon in the Pokémon Adventures manga.
Gastly is part of Agatha's Ghost army. They were used to attack the Kanto cities. Agatha also has her own Gastly outside of the army. The Gastly first appears as a dark fog lifting up Super Nerd Miles when he'd been beaten. Blue's Charizard was able to defeat Gastly by attacking out of nowhere, after which the boy deduces that the same Elite Four member whom he'd fought before was behind the scenes. In said battle, Blue's Scyther was used to Slash Gastly into pieces.
Arbok was sent out to battle Koga and Blue as it was their goal to try to defeat Agatha. Agatha's Arbok has several abilities activated by Agatha changing the patterns on its abdomen, including the speed boosting pattern, the attack boosting pattern, and the invulnerability pattern that repels poisons. However, its ability was nullified when its body was smeared with blood.
She is first seen as a Golbat spying on Red when he was getting on the Luxury Liner. After they boarded, Agatha transforms into a human and is very interested in Clefairy. She does everything to get Clefairy in obeying her. While on the ship, she causes the captain to collapse and makes Clefairy as the new captain. However, all of this turned out to be fake as Agatha only became interested in Clefairy was his health looked tasty. After sucking away Clefairy's health, Red finds out who she is and transforms into a Golbat. However, the health that Agatha drained from Clefairy turned out to not taste good and miniature versions of Clefairy escape Agatha's Golbat body. She was last seen in the water after the luxury liner got destroyed.
Agatha has the ability to transform into a Golbat. It first appeared when it was spying on Red and his Pokémon. It transformed into a human. Later, it was transformed back into a Golbat. It was last seen on a sunken cruise ship.
Though she claims to specialize in the Ghost type, the common type shared by all her Pokémon in the main series games (except Misdreavus and AlolanMarowak) is Poison. This is most likely because the only Ghost-type Pokémon in Generation I were in the Gastly line.
This makes her the only member of the Generation I Elite Four whose team is made up of Pokémon that all share a common type.