Mitsuki Kouyama

 ( Kōyama Mitsuki) is the 12-year-old granddaughter of Fuzuki Kouyama and the daughter of Aoi Koga and Hazuki Kouyama who dreams of becoming a pop singer and she has a tumor in her throat which not only restricts her ability to sing, but it will end her life in one year. With the help of Takuto Kira and Meroko Yui, the shinigami assigned to collect her soul, she is able to transform into a 16-year-old to fulfill her dream with her remaining time.

She was discovered by Seed Records and signed with them under her stage name "Full Moon" ( Furumūn). Masami Oshige (Mitsuki's manager) helped her through the beginning of career. Soon after her discovery she released her debut single, "Myself". Her second single, "Eternal Snow", was chosen from three potential songs. In the manga, Fullmoon's first and second singles are entitled "Angel" and "Missing Link". Her songs in the epilogue are "Smile" and for the final song called "New Future" (which is her first song in the anime).

Physical appearance
When Mitsuki was younger, she tied her hair as two pigtails that were straight and had bangs covering her forehead. As a twelve-year-old, Mitsuki is shown with dark brown "cork-screw" curled pigtails, a braided hairband above her bangs, two small strands of hair which reaches her chin let loose and straight and dark-brown eyes. When transformed into a sixteen-year-old, Mitsuki greatly resembles Hazuki Kouyama as her hair is wavy and brown, but she later on dyes it blonde that has one round bun on each side with each of her hair is taken into a strand and curled at the end excluding her bangs, but her eyes stay the same.

In the epilogue of the manga, Mitsuki has much longer hair where she ties it up in a ponytail that it looks closer to her 16-year-old hairstyle than her 12-year-old hairstyle, although her hair color is still dark brown.

Personality and Traits
Mitsuki is a cheerful and innocent young girl who loves nothing more than to sing with all her might. Even with her cancer gradually growing worse, Mitsuki says that she is more afraid of not being able to sing rather than dying, which proves her genuine love for music. She has many attachments to her family and friends, and those attachments are very important to her because she had been lonely after her parents' deaths when she was very young. Despite her happy-go-lucky attitude, Mitsuki also has a fierce temper, seen when she quarrels with Takuto Kira.

Early life
Mitsuki was born to Aoi Koga and Hazuki Kouyama, who married against the wishes of Fuzuki Kouyama. Hazuki died giving birth to Mitsuki. In the anime, her father died when she was a year old; in the manga, he died in a car accident while driving his wife to the hospital to give birth to Mitsuki.

She was moved from different foster homes until she arrived at an orphanage at age 5 (anime; 3 in the manga). As a child, she was quiet and did not have friends because she felt lonely without her parents. However, she soon met Eichi Sakurai, a wonderfully kind young boy who lived in the same orphanage. They became fast friends and spent all their time together. Mitsuki taught Eichi how to sing and he taught her about astronomy. However, as the years passed and their relationship grew, they fell in love with each other, however, Mitsuki failed to quickly realize this until he confessed to her days before he was adopted by a family in America. Before his departure, they vowed to achieve their dreams before they reunited. At 10 years old and wanting to confess her love to Eichi, Mitsuki was determined to make her dream of becoming a singer come true.

Soon after, Mitsuki's grandmother came to pick her up and bring her home. However, because she never approved of the child's parents' marriage, she ignored Mitsuki and forbid her from singing. As a result, Mitsuki had trouble feeling welcomed by her grandmother. After some time, she later found out that she had throat cancer, and the only cure was surgery; which had a possibility that she would lose her voice in the process. Mitsuki refused the surgery, saying she would rather die than being unable to sing again.

Storyline
When Mitsuki is 12-years-old, she is visited by two shinigami named Takuto Kira and Meroko Yui who tell her that she has one year left to live and they have come to take her soul when that happens. Mitsuki, who had been wanting to go to a singing audition but was rejected by her grandmother, rushed to the audition, desperate to fulfill her promise to Eichi Sakurai before her death. When Takuto and Meroko caught her, she pleaded with Takuto to help her in the audition, and she truly wanted to be a singer. Inspired by her genuine desire to sing, Takuto agrees, using his powers to transform her into a healthy 16-year-old. Despite her nervous debut, Mitsuki is able to win over the judges with her singing after remembering her promise to Eichi. She chooses the stage name “Full Moon” because it reminds her of Eichi, who loved astronomy.

As her singing career grows, Mitsuki faces numerous hardships, which includes jealous rivals, obsessed fans, the arrival of another, albeit cruel, shinigami named Izumi, and a blossoming love triangle between her, Takuto, and Meroko. Meroko, who is in love with Takuto, becomes increasingly jealous of Mitsuki but continues to help her with her problems. Takuto takes the role as her protector as he becomes drawn to her cheerful and determined personality, and becomes jealous of her love for Eichi, which she fails to notice.

Mitsuki's main focus to continue singing, hoping that her music will reach Eichi in America and he will realize her feelings for him. However, any chance of a reunion between the two is shattered when the shinigami discover that Eichi has been dead during the whole two years Mitsuki has been searching for him. In the anime, Mitsuki is offered an opportunity to go to America to meet a doctor who can perform the surgery without removing her vocal cords. She takes advantage of the trip and manages to locate Eichi's adoptive family, only to learn that he died in a car accident a few months after he moved. Despaired, Mitsuki goes to his grave, lies on it, and wants to die so she can go to the world he is at.

She then gives up on singing and her heath quickly declines, even causing her to consider suicide so that she can be with Eichi. However, Takuto's recollection of how much he regrets committing suicide prompts Mitsuki to come out of her depression and accept that Eichi would have wanted to continue with singing. In the manga, it is revealed that Mitsuki knew that Eichi has been dead all along; having watched the news about his plane crashing and seeing his name on the list of the deceased. Mitsuki said that she went into denial because the truth caused her intense pain, but she is forced to finally admit that Eichi is gone forever.

Eventually, as she struggle to adapt to life without Eichi, Mitsuki realizes that she is falling in love with Takuto after he confesses to her. Mitsuki is scared to accept his feelings because she thinks she will forget about Eichi. However, Meroko talks with her and explains how she fell in love with Takuto and Izumi, and tells Mitsuki that even though she will find another love, Eichi will always be special to her for the rest of her life because he was her first love. With that, Mitsuki is able to return Takuto's feelings, and he vows to never let her die.

In the anime, during her final concert, the day she was scheduled to die, she is saved by Takuto, thereby changing her fate. Afterwards, she has the surgery without the removal of her vocal cords, and is able to keep her voice. She is later reunited with Takuto with the help of Meroko. In the manga, Mitsuki discovers that the reason she can see the shinigamis is because Eichi's soul has been with her since he died. Takuto saves her from dying and disappears for three years. During this time, Mitsuki's cancer mysteriously disappears and she continues with her singing career. During a concert, she is reunited with Takuto and they begin a relationship, but Mitsuki will forever keep Eichi in a special place in her heart.

Aoi Koga & Hazuki Kouyama
Aoi and Hazuki are Mitsuki's parents who died when she was very young: Years later, she was taken in by her maternal grandmother. Although she doesn't remember her parents, Mitsuki seems to love them very much and treasures them, as they were the ones responsible for her love for music. Mitsuki had a difficult relationship with Fuzuki Kouyama, who disliked Mitsuki's father and blamed music for taking her daughter away from her. Eventually, Mitsuki realizes that her grandmother does care about her.
 * Hazuki died after giving birth to her
 * Aoi died soon afterwards.

Eichi Sakurai
Eichi is Mitsuki's first love who met in an orphanage when they were 5 and 11 (anime; 3 and 7 in the manga). They became fast friends, as close as siblings. As Eichi taught her astronomy, she taught him about music, and he encouraged her to be a singer. When Mitsuki was 10, Eichi confessed his love to her (even took her first kiss in the manga), and she was unable to reply due to being confused over the situation. However, when she heard about his move to America, she discovered her feelings for him, but rather than confessing, they made a promise to achieve their dreams before meeting again. Mitsuki's desire as a singer is to find Eichi and reunite with him. However, it is revealed that he has been dead all along (Mitsuki even knew about it the entire time in the manga while oblivious until she travels to America in the anime). Mitsuki goes into denial, then into a deep depression as she struggled to live without Eichi. With the help of Takuto and Meroko, Mitsuki is finally able to accept that Eichi is gone forever, but she keeps him in a special place in her heart.

Takuto Kira
Takuto is one of the shinigami assigned to take Mitsuki's soul when she is supposed to die from her cancer. However, he soon becomes her protector as he grows to admire her determination to be a singer, and eventually falls in love with her. it is later revealed that Takuto had played in a band with Mitsuki's father and even named Mitsuki when he was 12-years-old. In the manga, when she was 7 (and before he became a shinigami) Takuto actually met Mitsuki in her orphanage and promised to always protect her. Mitsuki was afraid to accept his feelings, even though she felt the same way, because she didn't want to forget about Eichi. Eventually, she was able to learn to deal with it and confess to Takuto and pursue a relationship with him.

Etymology

 * The name Mitsuki means "full" (満) (mi) and "moon" (月) (tsuki).
 * Mitsuki's surname Kouyama means "god" (神) (kou) and "mountain, hill" (山) (yama).

OVA

 * Full Moon o Sagashite: Cute Cute Adventure

Trivia

 * Mitsuki's last name is taken from the producer of voice actress Houko Kuwashima's radio "Club db"... without permission.
 * Mitsuki's eyes change color in manga illustrations according to Arina Tanemura's mood. But she confirms Mitsuki's official eye color is gray.
 * Mitsuki's hairstyle is taken from Kago_Ai|Ai Kago from the show called "Morning_Musume|Morning Musume".
 * The character model for Mitsuki is based on the author's assistant named Airi.
 * She looks like she's in grade school, but she's actually 19 years old.
 * Mitsuki has an alarm clock in the shape of a chicken.
 * In the manga, Mitsuki knew Eichi Sakurai was already dead because she saw a news report on TV about the airplane accident.
 * However, she didn't know until Episode 42.
 * In the manga, the reason why Mitsuki's illness wasn't as serious as Takuto Kira and Meroko Yui first thought is because she turned into Full Moon so much.
 * Meroko: "Hey... the reason why Mitsuki's illness wasn't as serious as we thought... do you think it could be because she turned into Full Moon so much?"
 * Izumi Lio: "Now you notice? It's because she spent less time as "Mitsuki". During her transformations into "Full Moon", her illness wouldn't get any worse."
 * In chapter 2 of the manga, her first single is stated to be released on "March 12".
 * As her birthday is in early May, she'd age to 13 very fast, near the start of the series.
 * Due to Takuto's magic, when her 16-year-old form wears off, she returns to her younger body.
 * Also in the manga, she had 1 full year left to live. That would mean by the end of that year, chronologically, she'd be very close to 14 years old, especially closer to when the main story (before the epilogue) ended.
 * In the epilogue chapter of the manga, "three years" have passed.
 * If Mitsuki was 14 by the end of the main story or not long after it, three full years later, she'd be around 17 or just over 17+.