Misty's Gyarados (Japanese: カスミのギャラドスKasumi's Gyarados) was Misty's tenth Pokémon; she adopted it in the Kantoregion after she returned home in Cerulean Blues. Its voice actor in both Japanese and English is Unshō Ishizuka.
Taking over the Cerulean Gym wasn't easy for Misty. Not only were there rumors of PIA inspectors arriving, an angry Gyarados was causing chaos. Misty tried many attempts to stop it but had to resort to locking it in a cage for a while. However, when the Invincible Pokémon Brothers got their Tentacruel to attack it with Poison Sting, Misty took the attack for Gyarados and earned its trust. Calmed, it finally allowed her to return it to its Poké Ball.
The capture of Gyarados is significant as Misty had been afraid of the species up until this point. This came from a traumatic event in her childhood when she had crawled into the mouth of a sleeping Gyarados. Getting it to earn her trust also led to her passing the inspection and truly earning Gym Leader status.
Moves used recently are in bold unless all moves all fit this case.
In the manga
In Electric Tale of Pikachu
Misty also has a Gyarados in the manga counterpart to the anime, but, unlike in the anime, it was the first Pokémon she ever caught. She trained it carefully until it evolved, due to this, it's extremely loyal to its trainer.
Misty's Gyarados, who was nicknamed Gyara, first appeared in Gyarados Splashes In!. It was stolen and used for experiments by Team Rocket, causing it to go berserk and attack Misty. Then Red appeared, recaptured Gyarados and returned it to her. After that, in You know... Articuno!, Red tried to get the HMSurf but he couldn't, so, Misty offered to trade Gyara with Red's Krabby, Kra, because Gyrara already knew that move. With Gyara he could reach Cinnabar Island.
Gyara seems to have a cheerful personality, as after it was caught by Red, it was constantly singing and moving. The other Pokémon seem to be afraid of it after it does this.
Related articles
For more information on this Pokémon's species, see Magikarp and Gyarados.