The guy who helped Mari, could he be Mari's ideal man?! Mari is super excited to have finally found her Prince Charming, but this guy, Chiaki, unfortunately has a dual personality! Due to some family circumstances, they start living together, when his mischievous personality appears and unfortunately snatches Mari's first kiss! Who is that man? What has she given him?!

Mei is a twelfth grader in an escalator school system. She has a crush on her upperclassman Takaoka; meanwhile, she has been annoyed by the pursuit of an eighth grader named Masami. However, at almost the same time she has found out that Takaoka already has a girlfriend while Masami has started to show impatience and begins to distance himself from her because being refused repeatedly, she suddenly realizes that her crush on Takaoka is merely an admiration and she has already fall in love with Masami.
(Source: AnimeNewsNetwork)

Everything was new to me: first love, first kiss, and first sex... I didn't know how to love you...
Also includes the short stories "Childhood Friends" and "Angel in the 21st Century".
(Source: M-U)

Side story in Boku no Hatsukoi wo Kimi ni Sasagu

From ShoujoMagic:

Yori's attitude has changed. He and his twin sister, Iku, used to be inseparable as children, but lately Yori's been different -- eager to be apart from Iku. Iku worries that he hates her, but the truth is far more scandalous. Now, Yori would risk losing everything to have his little sister. Iku loves her big brother enough that she would never risk losing him, and so she'll do what it takes to keep them together... but Yori doesn't want to have her that way -- as a sister dedicated to her twin. Can Iku ever love Yori not as a brother but as a man?

Boku wa Imouto ni Koiwosuru received the Shogakukan Manga Award for shoujo manga in 2008.

Shinobu is a typical high school girl...with an atypical hatred of boys. Long ago, she was horribly raped in a dark room by one and hasn't been able to tolerate being near them since. Now Yokomizo has appeared before her--a boy who looks exactly like the one who raped her in the past. He's the last guy she wants to have around, but then he confesses his love to her!?

Natsume has been engaged to Kasuga since they were six. After not seeing each other for ten years, she is so excited to see him only to find out how cold of a person he turned out to be. What’s more? Their fathers conspired to have them live together in one house and made them go to the same High School.

What will happen when, on the very first day, everyone found out?!!!!

This manga is comprised of three different stories:

1. Jesus! [Chapter 1-3] (Complete)
2. A Kiss on a flake of snow [Chapter 4-]
3. My prince charming

From Tenshi-Tachi:

Takuma and Mayu's story began when they were only 8 years old. During his childhood, Takuma was constantly hospitalised due to his heart condition. He soon became friends with Mayu, the daughter of the doctor in charge of his case. Day by day they grew closer and closer. However, the summer when Takuma was 8, he made a promise to Mayu that he couldn't keep... Their story unfolds as they grow older and their bond gets stronger...

This manga received the 53rd Shogakukan Manga Award for shoujo manga.

Extras:

? V.01 - Motto Ikitai...
There is a ghost looming in a hospital. Two little kids, Mayu and Takumi, are now scared after listening to the ghost story from a nurse. Was the story told by the nurse true or not? The kids decide to find out.

? V.07 - The Things I Want

? V.11 - Kare wa Ike mo Shinai Sarukoen wo Mezasu

Sawa is class president. Her teacher asks her to persuade Tatsuki, who always skips school, to actually attend classes. Tatsuki tells her that he will go to school only if she kisses him.
(Source: B-U)

Ogasawara Aki (age 25) is the reclusive, idealistic composer of the popular band Crude Play. Discontented with his life and with no drive to succeed, he meets Riko, the 16-year-old grocer's daughter in a band consisting of her childhood friends, and asks her to be his girlfriend on a whim. Riko, young and innocent, agrees immediately, but worries about her new boyfriend, whom she suspects has just broken up with a woman he cared very much for.
(Source: Nagareboshi Manga & Manga Abyss)