Misty

Misty is the gym leader of Cerulean City. Her starter was a Staryu, which is now a Starmie.

History
Misty is first encountered in Cerulean City, where she battles Daryon, Cameron, and eventually Evan for the Cascade Badge.

Starmie.
Starmie is Misty's first Pokemon, received as a Staryu. Starmie seems adept at using its body as sort of helicopter blades, spinning at rapid speeds and making it difficult to be hit.

Psyduck.
Misty appears to use her Psyduck primarily to set up, as shown by the fact that Psyduck's moves take effect later in battle, and she isn't usually upset that Psyduck is knocked out.

Trivia

 * Misty is the first gym leader that Daryon manages to defeat on his first attempt.