Lum is the female protagonist of the manga and anime series Urusei Yatsura. She is an Oni alien from the planet Oniboshi, where she was raised as a precocious and adventurous child. She attended primary school with Benten, Oyuki, and Ran, and the four of them became known for causing large-scale chaos. In junior high school, Lum, Benten, and Oyuki formed a small school gang. She was once engaged to a handsome Oni named Rei, but she ended the engagement because she found him dull.

Lum originally came to Earth as part of an Oni invasion attempt. She was chosen to play a game of tag against the human Ataru Moroboshi, with the fate of the Earth at stake. When Ataru caught her and yelled that he could now get married, Lum misinterpreted this as a marriage proposal and accepted. Because engagements are sacred on her planet, she has been determined ever since to be Ataru's wife. She moved into his home and enrolled at Tomobiki High School to be near him.

Lum is generally good-natured, sweet, and innocent. She is vivacious and uninhibited, and she is not afraid to show her feelings. She is fiercely loyal and devoted to Ataru, whom she calls Darling. However, she has a very quick temper and is extremely jealous. When angered, especially when Ataru flirts with other girls, she responds with powerful electrical shocks. Despite her quick anger, she does not stay angry for long and usually returns to her cheerful self once the threat passes. She is also very stubborn and determined, rarely giving up on any goal she sets for herself. At the same time, she is naive about many Earth customs, which often leads to misunderstandings and chaotic situations.

Her primary motivation is to be a good wife to Ataru and to build a happy home with him. She aspires to be the ideal wife and daughter-in-law, taking care of the house and helping Ataru's mother with chores. She is also motivated by her love for Ataru and her desire to keep him faithful to her, though her methods often backfire.

As the female lead, Lum drives much of the story's romantic comedy, as her forceful presence in Ataru's life creates constant conflict and humor. Her arrival on Earth and her relationship with Ataru are the central premise of the series. She is beloved by most of the male characters in Tomobiki but resented by many of the female characters, though she remains oblivious to the attention she receives from other boys.

Her most significant relationship is with Ataru. She considers him her husband and rarely leaves his side, though he often tries to escape her. She also has a close but strained relationship with her younger cousin Ten, whom she tries to keep out of trouble. Her friendships with Benten and Oyuki are strong, but her relationship with Ran is complicated by Ran's desire for revenge over childhood grievances. Her parents, Mr. and Mrs. Invader, appear occasionally as part of her alien background.

Lum does not undergo a dramatic change over the course of the series. Her fundamental personality and unwavering devotion to Ataru remain consistent. However, she gradually becomes more accustomed to life on Earth, and her interactions with Ataru occasionally show moments of genuine affection and care beneath the chaos. The friction between her and Ataru stems from their clashing personalities: she is naive and innocent yet strives to be a mature wife, while he is worldly but refuses to act mature.

Lum has the ability to fly and to generate massive electrical discharges from her body. These powers are tied to the two small horns on her head. If her horns are removed or if specially blessed yellow ribbons are tied around them, she loses her powers and becomes an ordinary girl. She is also skilled with advanced alien technology and often carries various gadgets on her person, though her attempts to use them for helpful purposes frequently result in unintended consequences. She drinks hot sauce like water and cooks extremely spicy food that is often inedible to humans.