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.

Most In the world of Anime, 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.

So that's why they trying so hard drinking much of milk or eating a lot of meat in order to increased the size of it.

1. 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

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 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.

4. Yuki Nagato - The Melancholy of Haruhi Suzumiya

Yuki is depicted as a closet otaku, shamelessly playing eroge games, cosplaying, even getting into a brief cosplay war with Mikuru before the latter concedes, and getting "excited" about anime and manga.

5. Hinagiku Katsura - Hayate the Combat Butler!

Hinagiku Katsura is one of the main heroine in the Anime Hayate no Gotoku.

She has long pink hair almost styled into a hime cut due to the sidelocks & bangs, and with a yellow hair clip and yellow eyes. 

This can be understood in the light of her interactions with Hayate after she fell in love with him: She acts tough in front of Hayate but at the same time, she also shows her vulnerability or her soft side like blushing, being shy - just like a normal teenage girl who has an innocent crush or feelings towards a guy.

Thus, it can be said that Hinagiku is a tsundere. Just wondering why flat chest characters are not far away from the word tsundere, hmm. 

6. Rin Hoshizora - Love Live! School Idol Project

Hoshizora Rin is also one of the nine main characters in Love Live! like Umi. She is 15 years old and a first year in Otonokizaka High School.

She likes to play sports, and she used to dress like a boy.

When she was still a child, she was often teased because of Rin appears to be a tomboy who like playing sports and not wearing a skirt when she was a child.

7. Kaede Kayano - Assassination Classroom

Kayano is a girl with a short stature, she has a little frame, hazel eyes, and green hair which is always fashioned in unique, cat ear-styled pigtails. 

Her inability to swim, her fixation on breasts, and her love of sweets, are traits she shares with Korosensei, though she had her breast fixation even before the implantation.

She dislikes people that are big-breasted, except her friend, Tōka Yada.

8. Kagura - Gintama

Although she doesn't like violence, she clearly use violence to solve her problems which Hasegawa Taizou labeled her as 'ultra hot (tempered/aggressive) China girl'. 

Kagura is also somewhat of a tomboy. She is often seen partaking in games with various neighborhood boys. 

Although her candid manner of speech often cause them to shun her, she doesn't seem to mind. 

In fact, she has no problem at all with forcing her way into their game. 

9. Yukino Yukinoshita - Yahari Ore no Seishun Love Come wa Machigatteiru

Yukino is a beautiful girl with waist-length black hair which is tied with two red ribbons by each shoulder and angular blue eyes.

She changed her hairstyle into twin tails when she went out with Hachiman and Komachi (but ended up with just Yukino and Hachiman) to buy a birthday present for Yui.

She is seen as flat-chested compared to most of the other female characters, but remains just as beautiful.

10. Riko Aida - Kuroko’s Basketball

Riko is so horrible at cooking that her teammates can't endure eating it. Moreover, she has the habit of mixing vitamins with her cooking.

She can also get jealous easily, as she was irritated by Seirin's attention towards Momoi and even punished them for looking at her.

Riko can be really scary when she's mad, and she's often teased as being 'flat-chested' compared to Touou's Momoi.

11. Nico Yazawa - Love Live! School Idol Project

Yazawa Nico is another one of the nine main characters in Love Live!.

Despite her looks, she is actually 17 years old and a third year in High School!.

Normally, Niko has quite an aggressive personality, but when she takes on her alternate personality she becomes a happy, upbeat character who is always smiling.

Nico predicted that her chest could grow 3 cm more. Therefore, she wrote 74 instead of 71 for her chest size

(source:animedition)