A large majority of the tanned or naturally dark female characters in anime are portrayed as airheads, sluts or both. An example is the brown-skinned Mizuho from Tenchi Muyo. Mizuho is a bumbling, bubble-headed Galatic Policewoman. She serves as a foil to her serious, level-headed light-skinned partner.
Pirotess from Record of the Lodoss War is a example of a dark-skinned female character who is made more sexualized than her light-skinned counterpart, Deedlit. Pirotess almost spills out of her scanty costume, and while Deedlit is protrayed as attractive, she does not expose as much skin as Pirotess. Urd from Oh My Goddess, is much more sexily-dressed compared to her fair-skinned innocent sister, Belldandy. There is Anthy from Revolutionary Girl Utena, who basically sleeps with everyone. She is a sympathetic character though, and the story presents psychological reasons for her sexual permissiveness. Topping the list is the objectified sex-slave Pink from the hentai title Dragon Pink.
This anime bias has its roots in common real-world stereotypes. In Japan. there is a stigma against women with tans, along the lines of them being airheads or sluts. In fact, there is a manga titled Peach Girl about a girl named Momo, who is discriminated against for having darker skin than other Japanese. Momo is a full Japanese, but she tans easily and is on the swim team.
The titular character of the series Nadia and the Secret of Blue Water is a tan, black-haired heroine who topped Animage magazine's fan poll for 'favorite female anime character'. The existence of a well-designed and well-received dark-skinned anime protagonist shows that animators can portray dark-skinned characters in a healthy, balanced way, but often choose not to for whatever reason.
Another example of a 'normal' dark-skinned female character is Janice from Princess Prince by Tomoko Taniguchi. Princess Prince is the story of Prince Lawrence and Prince Matthew. They're twins but unfortunately for Lawrence he looks exactly like his mother and his father has him live as Princess Lori. Janice is a side character who is treated pretty badly in the kingdom for being dark-skinned. She goes to bathe in a 'magic' lake hoping her skin color will change. Prince Matthew sees her and falls madly in love with her (which is out of character for him, since he's something of a playboy). In fairy tale fashion he invites her to stay in the castle with him. Janice eventually has to go home, putting their romance on hold.