Maybe you already know that anime girls character somehow are "not proud" for being flat-chested.
Practically, it's some kind of joke material to the other characters.

1. Nana Deviluke - To Love Ru

Nana Deviluke - To Love Ru

The first one is Nana Deviluke from To Love Ru. very known for has long pink hair and purple eyes like all Devilukeans.

FYI Nana actually has a smaller bust than Momo which the she is teased about by her younger half and she really hates, occasionally becomes upset and even hurt for being teased by her twins, Momo because of her flat chest. 

2. Umi Sonoda - Love Live! School Idol Project

Umi Sonoda - Love Live! School Idol Project

A beautifull yet flat chest character, Sonoda Umi. She is known as the one of nine main characters in Love Live!.

her age is still young at 16 years old, and is a second year at Otonokizaka High School.  

Umi is normally a shy girl, but she has a strong will and has some qualities of a leader.

3. Minami Shimada - Baka to Test to Shoukanjuu

Minami Shimada - Baka to Test to Shoukanjuu

Minami initially appears outspoken and violent, seen as when she often beats Akihisa up, but she is actually a shy and kind girl, while being self-conscious of her appearance.

She is a tsundere, someone who acts violent and uncaring on the outside, especially to their crush, but in the inside, is actually very caring and soft.

She is noted to be lacking in the chest area and is often called a 'pettanko'  or flat-chested, a word she is very sensitive to. According to Kouta and Akihisa, Minami's bust size is actually AAAA cup which makes the later gets beaten up by her.