Bokkuko (tomboy) characters in anime

These types of characters can be quite common in some shonen manga. Bokkuko girls are basically tomboys that serve as sidekicks and can also play the role of the female heroine along with their hero counterpart.

They can be physically slim, with short hair, skinny and flat chested, yet also have particular skills that you would normally expect from their male counterpart. (Shonen manga can be a bit mysoginistic when it comes to delimiting specific roles by gender, so don't shoot the messenger here XD)

At least we know that these delimitations by gender are changing.

Good examples would be Tatsuki Arisawa from Bleach, or Tenten and Sakura from Naruto.

These type of characters have evolved into strong female characters that don't necessarilly have to be the less appealing, quite the contrary, in fact.

Look at Yuzuriha from Saint Seiya: The Lost Canvas. In the original manga, she would have just been another side character as Marin and Shaina with little to almost no development. But she has a key role in helping her friends to achieve all the misions through Hades' Inferno.