Rich obnoxious jerk six-tentacled octopus musician who has his own, unnamed band (presumably of the marching variety) who first appears in the "Band Geeks" episode of SpongeBob SquarePants (September 7, 2001).
He is the longtime rival of main character Squidward dating back to band class in high school. In the episode he calls up Squidward, taunts him, and asks him and his band to fill in for him at a halftime show at the Bubble Bowl.
Squidward lies about having a band, and gets everyone in town to rehearse with him for the gig. Despite always sounding terrible, somehow the band becomes magnificent when it's performance time, and Squilliam faints dead away. Or maybe it's a heart attack, it's not clear, but just desserts are served.
He appears in several more episodes, but the only one with a musical context is the "Professor Squidward" episode where he's offered a job as a music teacher at the Bikini Bottom Prestigious Music College.
What instrument Squilliam plays is never shown, but it's probably a clarinet like Squidward.
Voiced by Dee Bradley Baker.