In the first film, Celia is celebrating her birthday, so Mike invites her to a Japanese restaurant called Harryhausen's (named in honor of legendary stop motion animator Ray Harryhausen), but due to Boo being in the restaurant, the place is contained by the CDA. One device that is made to make a large dome that would get rid of fatal germs, but could harm people without CDA armor, which is why Celia comes back to work the next day with serious injuries. She is initially upset with Mike and complains about how the date was ruined, but when she sees Mike in danger of being captured by Randall for devious purposes, and when Mike reassures his love for her, she saves Mike by subduing Randall with a crowd after she feign-announces him breaking the all-time scare record. She is seen again at the end of the film, having fully recovered and made amends with Mike, and tells him that a box full of magazines has just arrived. Both she and Mike are surprised to see that Mike has made the cover of the magazine. She kisses him and her hair tickles his face.
The year before the film takes place, she and Mike are supposed to have won the 7-legged race at the year's company picnic.
Celia's name may be a play on the word "Cecaelia." It might also refer to Scylla, a monster in Greek mythology known for snatching up those who strayed too close to her with mouths attached to tentacle-like limbs (somewhat like Celia's snakes).
According to the book Monsters Inc.: The Essential Guide, the five snakes serving as Celia's "hair" are named Amelia, Bobelia, Ophelia, Cordelia and Madge.
Celia makes a cameo in M.U. on a picture in the ending. Her phone number can also be seen.
It's implied that Mike may have met Celia sometime after getting a job at Monsters Inc.
Celia's age, according to the handbook, is probably 24, making her a few years younger than Mike.
Celia's appearance resembles to the Greek mythology legend, Medusa. They share some similarities and differences.
They both have snakes for hair.
Medusa has two eyes but Celia has one.
Medusa turns people into stone by looking into her eyes but Celia doesn't have this ability.
Celia is a nice monster compared to Medusa who is bitter.