SpongeBob and Squidward. SpongeBob is described as perpetually optimistic, and happy, cares for others, and loves his job whereas Squidward is unhappy, self-absorbent, hates his job, and is a Perpetual Frowner. This is perhaps the reason why Squidward was appointed as the manager over SpongeBob in The Movie, since he was more mature for the job while SpongeBob acts childish despite being a legal adult.
Patrick to SpongeBob. They're both ditzy Manchildren who share such interests as jellyfishing and bubble blowing, but while SpongeBob is something of a Neat Freak, a Workaholic able to hold down a steady job and often proves to be The Ace at whatever he tries (except driving), Patrick is a Fat Slob, a Lazy Bum who never stays employed for long, and is shown to be so incompetent he can't even sweep or open a jar without outside help, but can drive surprisingly well.
Mr. Krabs to Squidward. While both of them have no tolerance for funny business, have good hearts every now and then in spite of their jerkassery, respond to SpongeBob's childish behavior with sarcasm, and have gone as far as to plot fatal schemes to get him killed, they're different from each other in that:
Mr. Krabs is considerably armed and served in the Navy, ultimately justifying his power to take down various antagonists.
Squidward is emotionally sensitive and insecure when in private, becomes timid whenever he suffers a horrible fate, and has become an exploit of Krabs's horrible schemes himself.
Sandy to SpongeBob. While both of them are dorky, energetic and eccentric Ridiculously Cute Critters who like karate, their personalities differ in that:
SpongeBob is a naïve and kooky Ditz who's childish, pestering, wimpy, cowardly, and generally ignorant in all but a few topic areas,
Sandy is a mature, tough, bold, and intelligent individual who more often than not plays the Only Sane Woman role.
