Tachibana Oto is a 15 year old orphan from Tachibana House. She decides upon entering high school that she wants earn her own school fees. In order to do so, she takes on the job at her new school as a live-in dormitory maid for "Class Z", an elite class full of first-year students with special skills and personalities, while also attending classes at the school. Oto starts her new life away from her family in the school dormitory, surrounded by all these (mainly) good-looking boys. Among them is Goryuukanda Jin, the dormitory head, a boy who is amazingly strong and who keeps a pet shark(!) at the dormitory's swimming pool. The two volume manga of Milk Crown contains charming and funny story revolving around the daily lives of the people in the dormitory, and the mysterious relationship building between Oto and Jin, which continues in the two sequels, Milk Crown H and Milk Crown Lovers.
Oto is a caring, cheerful, and kind girl, but at times, can be rather stupid. She is an orphan from Tachibana House. She has been there for a long time, and considers the other kids at the orphan as her younger brothers and sisters (and they all adore her). When Oto enters high school at the age of 15, she decides to take a job at her new school to earn her own school fees. She applies as the new live-in dormitory maid for the special class Z, even though she is from class Q. Oto adapts easily into the new dormitory life and befriends the class Z students from the beginning. She slowly discovers each student's special talents, and they all earn her admiration.
The most trivial character that attracts Oto's attention is Jin, because he doesn't have a noticeable special talent. She tries a series of attempts to discover his talents, but to no avail. Oto has met Jin a long time ago, but she has no memory of their encounter. When Oto was really young, she ran from the orphanage. She was lost and hungry, so when she saw Lady, Jin's pet shark, Oto attempted to her. Jin came to Lady's rescue. They became friends on the spot, and Jin made a promise to marry Oto when they grow up. But because she had a fever, she'd forgotten about the proposal. And so Oto falls in love with Jin, without realizing that he is her childhood fiance.
Jin is a 16 year old, first year student of class Z. He is also the dormitory head. Besides the good looks, Jin is fun, kind, humourous, but a dare-devil. When he feels devilish, out come the fangs. He is very adventurous and a risk-taker. He welcomes Oto to class Z by luring her to the rooftop of the dormitory, and jumps off the building with her into the swimming pool below. Jin also has a pet shark, Lady. He often allows her to stay in the dormitory's swimming pool, and sometimes forgets her in there. Lady had been in the swimming pool during Oto's welcoming ceremony, and had lots of fun chasing Oto fifty laps around the pool.
Jin is also Oto's childhood fiance. Once when he was really young, he lost Lady (then a baby) at the beach. He eventually found Lady with Oto. Oto had ran away from the orphanage. She was lost and hungry, and attempted to eat the poor shark. Jin came to both their rescues. The two children became friends quickly, and Jin made a promise to marry Oto when they grow up. Even though Oto has no memory of the encounter, Jin never forgotten about his promise. He finds out early that Oto is his childhood fiancee, and tries too subtly to help Oto regain her memory, but at the same time, trying to keep his cool. Jin is also very good at martial arts (he likes to pick fights) and comes from a wealthy family.
Yamato is also a first year student in class Z. He is a pretty boy, though friendly and kind. He is the number 1 pretty boy in school. His special talent is performing. He, along with Nagi, are high school idols. They perform in television drama shows. He likes being an idol, but doesn't quite enjoy the attention he receives from girls. Yamato can be scary at times. It appears that he has the power to summon spirits, and he uses this special skill to scare Oto. He is also really good at martial arts. He only performs martial arts when he gets really angry and goes into evil mode. Yamato is also really smart, smarter than he looks. He could tell that Jin was planning from the very beginning to regain Oto's memory.
Nagi is very similar to Jin in character. He is honest, quiet and somewhat mysterious. He likes to keep to himself and doesn't speak much, although he can be quite fun sometimes. Nagi is also a first year class Z student and an idol. He is Yamato's childhood playmate and partner as idols in television acting. They named themselves Izanagi. The two of them are actually quite famous with the girls at school. They even have a fan club. Nagi first love was Yamato. When he was younger, he really believed that Yamato was a girl. Because he was weak, he couldn't leave the house. His love for Yamato leaded him to tell Yamato "Let's elope together!" No one knows what happened after that.
Ukyou is a first year class Z student who is a fashion designer. He is very cheerful, sweet, and a little crazy. He is also somewhat feminine, and may be easily mistaken as gay. He likes to tell people that his heart is that of a woman and that he loves Jin, because Jin was the first guy to admire Ukyou's talent. His special talent is his ability to sew. He loves to design and make cute clothes for cute girls, especially Oto, who is just as eager to try on his new clothes. Ukyou has a really close relationship with Oto. Whenever Oto is thinking deeply, she often plays with Ukyou. He always find himself as the substitution for the fish during dinner preparation or hanging on the laundry line with the rest of the laundry.
Toki is Jin's fiancee. Toki's and Jin's families are very close business partners. Their parents arranged for their marriage, but they are only pretending to be engaged. Jin and Toki are very close childhood friends. But since the day Jin stood her up on their formal wedding ceremony, Toki has also proclaimed Jin as her biggest rival. Toki is also a member of class Z, and is very talented at... everything. Basketball, cooking, tea ceremony, anything you can name. She is not only talented, she is also very pretty. Toki is a fun, open-hearted and cool girl. She tries to play with Oto at the beginning by not telling her that the engagement is a lie, but Toki ends up as a big supporter for Jin and Oto.