A "Pettanko" character is a female who is flat-chested or has small breasts and is either concerned by it or proud of it.
In a world where large breasts are generally more popular amongst men, girls who are less-developed (either having particularly small breasts or being completely flat) often feel concerned about their small size. In some cases though, women are proud of their petite frame, for several reasons, one of which often being that some men prefer a petite woman to one with particularly big boobs.
It should be noted that not all small-breasted females are Pettanko characters, as they must also harbour strong feelings of either resentment or pride towards their small chest.
There are two basic types of Pettanko:
Sometimes characters switch mind-sets on the subject. Usually when a type 1 Pettanko finds out that a male character dislikes big breasts, they suddenly become proud of their bust and will likely try to draw extra attention towards it.
Fans who prefer flat-chested anime characters sometimes refer to them as "Delicious Flat Chests" or DFCs.