Deb is a blue/black and white striped Humbug damselfish who lives in the dentist's fish tank in Sydney, Australia. She seems delusional, as she thinks her reflection is her sister named Flo, who may or may not exist. She eventually escapes with the rest of the tank fish into the ocean. During the credits, since Deb is no longer in an artificial environment surrounded by glass walls, she now admires her reflection in a bubble until a credit text pops it.
Deb's name and her imaginary sister's together allude to "ebb and flow", the movement of water, often in relation to natural tides.
Eventually, Flo became the name of a character in the Cars series.
According to the credits and other media, Deb is listed as "Deb (and Flo)".
Deb could have actually had a sister named Flo back where she used to live.