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Spongebob Squarepants (voice: Tom Kenny) ~ the star! He is always optimistic and works at the Krusty Krab, home of the Krabby Patty.
Spongebob's mom and dad (voices: Mary Jo Catlett and Tom Kenny) ~ they live outside of Bikini Bottom and come to visit their son occasionally. The square gene must have skipped their generation and been passed on to Spongebob, because they are both round like natural sponges.
Spongebob's grandma (voice: Marion Ross) ~ a caring, sweet lady who always gives Spongebob fresh-baked cookies, a story, and sweaters with love in every stitch. Watch out for her very pink, long-lasting kisses!
Patrick Star (voice: Bill Fagerbakke) ~ a dopey, lazy, unemployed starfish whose schemes always seem to get him and his best buddy Spongebob in trouble.
Squidward Tentacles (voice: Rodger Bumpass) ~ surprisingly, an octopus. On the Sleepy Time episode commentary on the Tide and Seek DVD, Stephen Hillenburg mentions that he is an octopus, and when people ask why he only has 6 appendages instead of 8, he responds, "That's probably why he's angry." =) He is boring, plays the clarinet very badly, and constantly feels annoyed by Spongebob and Patrick's antics. He works at the Krusty Krab with Spongebob.
Mr. Krabs (voice: Clancy Brown) ~ Spongebob and Squidward's boss and owner of the Krusty Krab. He loves money and is always stingy about it, but he truly cares about his two employees.
Sandy Cheeks (voice: Carolyn Lawrence) ~ a Texan squirrel who needs a pressurized suit and her own private air-filled dome in order to live underwater. She and Spongebob both love karate.
Gary (voice: Tom Kenny) ~ Spongebob's very intelligent pet snail who meows like a cat.
Plankton (voice: Mr. Lawrence) ~ a mean, vertically-challenged invertebrate who owns the Chum Bucket restaurant, where business is not booming. He is always trying to steal the Krabby Patty secret formula from Mr. Krabs, but he hasn't succeeded yet.
Larry (voice: Mr. Lawrence) ~ a buff, sporty lobster who spends most of his time at Goo Lagoon, the underwater "beach" in Bikini Bottom. In the episode Ripped Pants, Spongebob was a little bit jealous of how Sandy wanted to hang out with Larry, but Spongebob finally learned that he just had to be himself. Larry is not to be confused with Lary, a grouchy snail that Spongebob used in the episode Dumped to replace Gary.
Pearl (voice: Lori Alan) ~ Mr. Krabs's teenage daughter. Of course, it doesn't matter that he's a crab and she's a whale. She is pretty much the only thing Mr. Krabs loves more than money. Spongebob went to the prom with her when "Mr. Long, Tan, and Handsome" stood her up.
Mrs. Puff (voice: Mary Jo Catlett) ~ Spongebob's boating school instructor. Spongebob has taken his driving test over 38 times, and he still hasn't passed. Whenever Mrs. Puff collides with something at a great speed, she inflates (she's a blowfish).
Mermaidman and Barnacleboy (voices: Ernest Borgnine and Tim Conway) ~ Bikini Bottom's resident superheroes. They are retired, living in the Shady Shoals rest home, but they still have a little bit of pizzazz in them.
jellyfish ~ this isn't one specific character, but you can find many of these creatures in Jellyfish Fields, just outside Bikini Bottom. Spongebob and Patrick's favorite hobby is jellyfishing, where they catch jellyfish and let them go.
anchovies ~ again, not just one lives in Bikini Bottom. The most common thing they say is "meep". In Help Wanted, the first episode, they invaded the Krusty Krab, and Spongebob quickly whipped up an enormous amount of Krabby Patties to feed them, saving the restaurant.
the doctor fish (voice: Rodger Bumpass) ~ Bikini Bottom's loyal physician who has appeared in Suds, I Was a Teenage Gary, Patty Hype, Squid's Day Off, Band Geeks, The Bully, No Weenies Allowed, I Had an Accident, Born Again Krabs, and other episodes. However, he's a bit squeamish because he couldn't give Gary his shot of snail plasma when Gary was ill.
Scooter (voice: Carlos Alazraqui) ~ the fish that I used to call "the aw-aw-aw-aw-awesome dude"! This lavender-colored surfer fish died in Bubble Buddy, but he came back in Squirrel Jokes, a later episode. In Walking Small, Spongebob turns himself into a surfboard for the dude to use, and after going surfing, he tells Spongebob, "Thanks, dude! That was aw-aw-aw-aw-awesome!" (He's my favorite character.) =)

additional voices:

Painty the Pirate (from opening credits): Pat Pinney (but Painty's live-action mouth belongs to Stephen Hillenburg) =)
Patchy the Pirate: Tom Kenny
Potty the Parrot: Stephen Hillenburg
French Narrator: Tom Kenny
Flying Dutchman: Brian Doyle-Murray
Karen (Plankton's computer wife): Jill Talley
Squilliam Fancyson: Dee Bradley Baker
Manray: John Rhys-Davies
The Dirty Bubble: Charles Nelson Reilly
Old Man Jenkins (a varying character): Tom Kenny, Mr. Lawrence, Dee Bradley Baker
Bubblebass (from Pickles): Dee Bradley Baker
King Neptune (from Neptune's Spatula): John O'Hurley
Grubby Grouper (from Bubble Buddy): Corky Carroll
Kevin (from I'm Your Biggest Fanatic): Dee Bradley Baker
Monty P. Moneybags (from Artist Unknown): Mr. Lawrence
Flats the Flounder (from The Bully): Thomas F. Wilson
Reg (from No Weenies Allowed): Thomas F. Wilson
the D.J. (from Krab Borg): Rodney Bingenheimer
the "CHOCOLATE!" fish (from Chocolate With Nuts): Mr. Lawrence
the Strangler (from Spongebob Meets the Strangler): Thomas F. Wilson