She seems to be a sweet little airhead, but don't let it fool you for a second. Ran is not a nice person, she's mean-spirited, crafty and hell-bent on revenge.
Ran was Lum's childhood friend on planet Uru. As kids they were an inseparable pair. But one day when Ran came to earth, Lum soon discovered that she had obtained a frightening split personality and a bitter hatred towards our green-haired heroine.
This all started way back many years ago. As children, Ran was always being punished for Lum's mistakes and was always getting a jipped indirectly because of Lum. But these were just leading up to what drove Ran over the edge. Both Ran and Lum were interested in same guy; that handsome shapechanger, Rei. Ran was completely crazy about him, but Rei only had eyes for Lum. Then one day Rei and Lum got engaged and Ran was crushed. Hearing that he got dumped soon after didn't help any either. As Ran pined over all that had gone wrong in her life, she became twisted and bitter, and began to see Lum as the source of all her problems.
A while later she heard of Lum's engagement to an earthling and decided that it was time for revenge. Lum stole Rei from her so it was only fitting that she steal Lum's "Darling." Ran came to earth disguised as a human. Her plan was to steal Ataru from Lum or more precisely, she was going to steal his youth. Ran's kiss has the power to take away a man's (or even woman's) youth. She sucks it right out of their lips. And that's exactly what she planned to do to Ataru. To make sure Lum wouldn't blow her cover she threatens to tell everyone about the time she slept over and wet when the bed when they were children. This keeps Lum from blabbing about Ran being an alien, but she tries as best she can to keep Ran's plan from being realized. Lum's work was cut out for her since Ran can be extremely deceptive and Ataru is really intent on giving Ran a kiss.
But Ran doesn't rely only on her power and feminine charm to get what she wants. She's also been known to use black magic heavy artillery, a plethora of alien oddities, and her master cookery as means to extract her revenge. Ran's primary target is Lum, but she is usually a mean little witch to anyone and everyone who gets in her way.
Ran behaves all cutsie-cute and innocent, but it's all an act. Under that girlish facade she's cynical, mean, vengeful, maniacal and obsessive. The cuteness act was one part of her plan to get revenge but after doing it for so long it had become ingrained in her personality. The two sides of her personality constantly compete for dominance. Because of that she switches from psycho-bitch to airhead and vice-versa rather suddenly and frequently.
As the series moves on and Ran's bitterness towards Lum wavers, until they once more become friends. Time heals wounds, but also she mellows out because Rei and Ran become closer later on in the series. Rei begins to pay more and more attention to Ran and less to Lum. But it would be more accurate to say that Rei's attracted to Ran's cooking rather than Ran herself.
Notes about the name
Ran has two meanings, both of which are suited to her warped personality. One meaning can be taken as "orchid," the other as "civil war."
Voice: You Inoue
Early on, Ran's voice was done by You Inoue who is the voice of Kanuka Clancy from Patlabor. She was later replaced by Kazue Komiya. But You Inoue did return to UY as the voice of Karla in Urusei Yatsura Kanketsuhen