who are the other characters in it?
Henry J. Waternoose III, James P. "Sulley", a small girl, Celia Mae and others.
Monsters, Inc.
 Henry J. Waternoose III, the company's CEO, is determined to find a solution to the problem. James P. "Sulley" Sullivan and his best friend, Michael "Mike" Wazowski, are the organization's top employees, but their chief rival, Randall Boggs, is close behind.

One evening after work, Sulley discovers that someone has left a door activated on the scare floor. He opens it and a small girl has entered the factory. After several failed attempts by Sulley to put her back, Randall sends the door back into the factory's door vault and Sulley conceals her and takes her out of the factory. He inadvertently interrupts Mike's date with his receptionist girlfriend, Celia Mae,
who does mike's voice in monsters inc Billy Crystal.
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Monsters, Inc. [SEP] Plot
Before Randall can use the machine on Mike, Sulley intervenes and takes Mike and Boo to the simulator where scare recruits do their training. But Waternoose was expecting Sulley to give the recruits a demonstration and forces him to roar, which scares Boo. Mike tells Waternoose about Randall's plan. But Waternoose, who is secretly in league with Randall, exiles Mike and Sulley to the Himalayas instead. The two meet the Abominable Snowman, who tells them about a nearby village, which Sulley realizes he can use to return to the factory. Sulley (who by this time, has realized the true effects of scaring on children) prepares to return, but Mike refuses to go with him, caring more about their dream of breaking the all-time scare record and feeling betrayed when he decides it no longer matters.