Calvin Clifford "Buddy Boy" Baxter
The protagonist of the story. A worker of the Consolidated Life insurance company, he lends the titular apartment to his superiors for their personal parties in order to rank positions in the company. He is also secretly in love with Fran Kubelik, one of the building's elevator ladies.
- Hero with Bad Publicity: His neighbors, especially the Dreyfusses, think he holds wild parties in his apartment when it's actually his bosses who bring drinks, beautiful girls and put on loud music. In a somewhat hilarious way, he has a much worse reputation when Mrs. Dreyfuss thinks Fran almost committed suicide because of him instead of Mr. Sheldrake.
Frances "Fran" Kubelik
An elevator operator at the Consolidated Life insurance company. She has a secret affair with Jeff Sheldrake, the building's staff director, that is trembling her feelings.
- Break the Cutie: Fran is a very kind and cute woman on the outside, giving compliments to C. C. Baxter which drives him crazy. But she is in love with Sheldrake who is causing trouble to her personal life.
- Broken Bird: Completely and utterly destroyed on the inside. Appropiately symbolized with her broken portable mirror.C. C. Baxter: The mirror... it's broken.Fran Kubelik: Yes, I know. I like it that way. Makes me look the way I feel.
- Driven to Suicide: Fran took an overdose of Baxter's sleeping pills and fell unconscious on the verge of death. Luckily, Baxter and Dr. Dreyfuss saved her life in time.
- Love Hurts: Fran is in love with Sheldrake, even though the man is only using her for his own pleasure.
Jeff D. Sheldrake
The personnel director of the Consolidated Life insurance company building in New York. He is a married man that has a secret affair with Fran Kubelik, but he doesn't respond to her romantic feelings.
- Affably Evil: He seems a comprehensive and cool guy, but only uses Baxter to employ his apartment for his own like the previous boses before him.
- Asshole Victim: At the end, C. C. Baxter won't lend his apartment to him ever again for hurting Ms. Kubelik's feelings, he gets divorced when Miss Olsen tells her about her affair to be with Fran and she leaves him in New Year's Eve.
- Bad Boss: In a manipulative way. He only wants Baxter's apartment for his own amusement. And even they are lovers, Sheldrake is still Fran's boss as well and he is hurting her feelings.
- Big Bad: He is responsible for not responding to Fran's feelings, hurting her with fake promises and a very distant attitude.
- Your Cheating Heart: He spent nights with other women for a long time discretly for fun.
Dr. Dreyfuss & Mildred Dreyfuss
Baxter's elder neighbors. Mr. Dreyfuss is also a doctor.
- Black Comedy Burst: Dr. Dreyfuss asks Baxter if he could donate his body to the medicine faculty in case he dies.
- Cool Old Guy: Dr. Dreyfuss warns Baxter every time he has "parties" as it could give him back as a punishment to his health and life. He also immediately arrived to save Fran's life when Baxter asked him for help and he reconsidered his duty as a favor to a neighbor. Also, he was the one who taught C. C. Baxter's most important lesson in life: be a man, a "mensch".
- Death Glare: Mrs. Dreyfuss gives a few ones to Baxter when she was told that Fran committed suicide because of him.
- No Name Given: Dr. Dreyfuss' first name is never mentioned during the whole film. However, the opposite happens to his wife whose name is Mildred.
Sheldrake's personal secretary and one of his former lovers.
- Jerkass Has a Point: She told in Fran's face that Sheldrake is only using her for his own amusement in a quite aggressive manner at the office's Christmas party. It's true, Sheldrake had several lovers in his life and made lots of false promises, but the way she told it to Fran made her day a true mess. Also, she might as broken and frustated as Fran.
Al Kirkeby, Mr. Vanderhoff, Mr. Eichelberger and Joe Dobisch
C. C. Baxter's superiors in the building and the ones who praise him to rank positions when Baxter lend them the apartment for their personal parties.
- Dirty Old Man: All four seem to have reached their fifties and like to spend nights with younger women. Most notably when Kirkeby touches Fran when leaving the lift.
- Karma Houdini: Unlike Sheldrake, they don't receive any punishment for using Baxter's apartment abusively.
- Moral Myopia: Kirkeby tells Karl, Fran's hot tempered brother-in-law, that Fran spent Christmas at Baxter's apartment to get some revenge for not lending it to him.
- Your Cheating Heart: A few of them are married such as Kirkeby.
Fran's brother-in-law, a NY cab driver.