Lorelei's Cloyster is one of her main Pokémon in Pokémon Adventures. It first appeared along with her Dewgong in Do Wrong, Dewgong!. Cloyster traps Yellow and Bill in a cave by collapsing the entrance with a Spike Cannon. When Yellow tries to escape, Cloyster sends an Ice Beam-powered Spike Cannon which stops them. Eventually, they do escape and Yellow attacks with Pika, with it using Thunder Shock. Lorelei tells Cloyster to Withdraw into its shell and Pika's ThunderShock bounces off harmlessly, though it became too stunned to move.
After a long chase, Lorelei gets Cloyster to Aurora Beam Yellow and Bill, freezing them in place along with an entire tidal wave. Then, Lorelei tries to extract information from Yellow by getting Cloyster to bombard them with its Supersonic, Yellow's Rattata comes and rescues them at the last minute with its Super Fang.
In Valiant Venomoth!, Lorelei appears again, this time riding in her Cloyster, to battle Green and Sabrina. She uses voodoo dolls made from her Jynx to freeze their hands together and present the threat that any part of the dolls that are destroyed means they will, too. Cloyster then uses Ice Beam on them, but it barely misses them. Green sends out Nidorina and Sabrina uses Venomoth. Nidorina charges at Cloyster and Venomoth uses Sleep Powder. Cloyster uses Blizzard to blow the Sleep Powder into Nidorina's face which puts her to sleep. Venomoth charges again and again, with Cloyster withdrawing each time so that all the attacks don't hit. Eventually Cloyster falls over, fainted, and Lorelei then realizes that it had been using Mega Drain the whole time.
When Lorelei later reappeared in the FireRed & LeafGreen chapter, she uses Cloyster again to defeat countless Unown with another Spike Cannon, but is taken by surprise when Sird's Starmie's use of Camouflage takes advantage of this opportunity to approach them whilst disguised as part of the terrain, and a Hyper Beam knocks out both it and its Trainer.
Personality and characteristics
Cloyster has no problem in following Lorelei's commands as shown when she relied on it in order to do dangerous tasks and to prevent anyone from interfering with her plans.
In The Mandarin Island Miss Match, Prima used Cloyster against numerous opponents in a tournament and defeated them all. Eventually, Ash challenged Prima to a battle. They agreed to use one Pokémon each, and Prima used Cloyster against Pikachu. Pikachu used a Thunderbolt, but Cloyster Withdrawed into its shell and the attack hit it harmlessly. Pikachu then used Agility, but Cloyster used Reflect and Pikachu got thrown backwards. After getting back on his feet, Pikachu used Thunder which hit Cloyster, who started to glow red. Pikachu tried to use Quick Attack, but was defeated by an Aurora Beam-Take Down combo, causing Ash to lose.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
In other manga
Cloyster in Pokémon Zensho
Cloyster in Pokémon Pocket Monsters
In the Pokémon Pocket Monsters manga
Lorelei's Cloyster debuted in To The Pokémon League, and it is Cloyster's only appearance. It first appeared along with Lorelei's other Pokémon when Lorelei welcomed Red to her school. Jynx used it as a weapon by smacking Clefairy. Later, it was snuck in Clefairy's food bowl, much to his ire. When Lorelei revealed herself to be an Elite Four member, Cloyster, along with Lorelei's team, had a battle with Red. Cloyster attacked Clefairy in which Clefairy dodged. Since Clefairy dodged, Cloyster crashed into a wall. It became the second opponent to be knocked out.
In the Pokémon Zensho manga
During Satoshi's time in the Pokémon League, Lorelei sends out her Cloyster. Cloyster goes up against Satoshi's Raichu who has the type-disadvantage against the Electric-type. It wasn't long before Cloyster gets taken out by Raichu's Thunderbolt, advancing Satoshi to the next room. Cloyster was not seen using any moves due to its brief appearance.
In the games
Lorelei uses Cloyster in every game she appears in, being the only one to do so.