Hajime is a tall, fair-skinned, well-built young male, with neck-length black hair and purple eyes. In his initial setting, Hajime had a lock of hair highlighted purple. His theme color is violet.
Hajime is known to be of commanding presence, making him stand out compared to other people. With his naturally great amount of charisma, he often ends up being the center of the group without him realizing. He is hailed as if he were a "King" by many, not only because of his personal appeal, but also because he is good at taking care of those around him. While he is a man of very few words, he can be quite blunt, at times sarcastic and severe, but well-meaning. He also maintains a dislike for doing things half-done or half-way.
In addition to his strong aura, he also possesses high intellect, as he was a the top student among those that attended a very elite all-boys private school that had been hailed as one of Japan's top schools. He also possess a high level of English proficiency.
Hajime also possess a lot of physical strength, often manifested in his famous "Iron Claw" technique, which makes him strong enough to crush an apple with his bare hands. He has also practiced the art of self-defense, and has the strength of break a certain number of tiles.
Despite his repertoire of skills and intellect, as well as his family background, Hajime keeps himself grounded and does not let himself get carried away with what he has accomplished.
Surprisingly, he also has a childish side, which usually manifests when he and the senior members assemble. He can also sometimes be rather airheaded, making him the target of teasing by others.
He has soft spots for animals and people younger than him. He often help them study by holding study session which receives the name "Iron Claw Cram School" because he's known to be strict to the ones he taught and punish them with his infamous iron claws.
He and Haru first spoke with each other sometime after they're assigned to the same class on their second year as junior high school students; Haru had been playing with others while Hajime had been taking a nap.
Age: 17 years old
Height: 182 cm
Hajime is in his third year of high school, and is, among other things preparing for his university entrance exams. On his way home from school, Hajime finds an unconscious large black rabbit at a park near a train station. Unfamiliar with animals himself, he calls close friend and classmate Yayoi Haru and asks for his help. In their attempt to find help for Kuroda, they encounter a suspicious man, who turns out to be the rabbit's owner who scouts them as soon as they meet.
He and Haru are the first among the newly scouted idols to move into the Tsukino Dorms, and one day, encounter Shiwasu Kakeru, Kisaragi Koi, Uduki Arata, and Satsuki Aoi, who are all looking for the Tsukino Dorms. Haru recognizes the four as the remaining members of the idol unit they will form together, and accompanies them to the Tsukino Dorms.
Sometime within the year, their planned sibling/rival unit Procellarum, is officially announced, although he and Haru had been well aware of Tsukino Talent Production's plan to line-up a second unit to engage in idol activities with and against their unit even before Procellarum's official announcement.
Age: 18 years old
Height: 182 cm
Hajime graduates from high school during March, and started his first year of university studies in April.
More or less a year has passed ever since their debut. Their jobs as both individual idols and as a unit have begun increasing, quite the opposite from their initial year that focused more on practice and training. Hajime particularly has a lot of solo jobs, such as a promoter for a cosmetics brand aimed towards women.
Age: 19 years old
Height: 182 cm
Hajime starts his second year of university in Spring 2015.
Six Gravity and Procellarum also hold their first collaboration live, with all their songs featured in their second set of solo CDs, together with their cross talks in-between solo performances during their collaboration live. He and Procellarum's leader Shimotsuki Shun are the last to release their CDs, releasing them in November.
Age: 20 years old
Height: 182 cm
Hajime turned 20 years old in January, making him a legal adult. Starting Spring this year, Hajime will be starting his third year of university studies.
Age: 21 years old
Height: 182 cm
Hajime starts his fourth year of university in Spring 2015.
Age: 22 years old
Height: 182 cm
Shun is a big fan of Hajime. While Shun may be noisy and troublesome compared to Hajime, the two leaders get along quite well.
Although his family lets him do whatever he wish as long as he will be responsible with his actions, they seem to want him to attend the family's stiff family gatherings. Hajime had been known to escape from those gatherings for many years.
Hajime and Yuki are distant relatives. They shared the same black hair and refined air about them that, when they are together, makes ordinary people find it difficult to talk up to them. They seems to see each other on the same level and has a friendly relationship, engaging in conversations whenever they meet on occasions like family gatherings. Yuki is also on good terms with Hajime's mother. 
- His name reflects many of his traits:
- His surname, Mutsuki (睦月) is the archaic name for the first month of the lunar calendar; poetically, January.
- Hajime may come from the noun hajime (始め), meaning "beginning", "first (in line, etc)"; from verb hajimaru (始まる), meaning "to start", "to begin (anew)" as January (surname) is the start of the year.
- Similarly, his official fanclub also reflects his position as a January representative:
- His (and Yuki's) official fanclub name, "initium", is Latin for "beginning", which reflects his given name.
- A common motif between the fanclub logo and the crest is the butterfly, which symbolizes transformation and metamorphoses in Japanese culture. Noble Japanese families also often choose the butterfly to represent their families, which reflects Hajime's belonging to the noble Mutsuki family.
- He is right-handed.
- His eyesight (both eyes) is 1.2, which Koi happily found "normal".
- He likes his eggs sunny-side-up with soy sauce; the same goes for his ramen preferences.
- He likes both sweet and spicy foods.
- Chisa was the one who gave him the nickname Hajiking.
- He received the nickname 'Heartbreak Purple' because a magazine for 2D idols once held a popularity enquette about the songs and three Tsukiuta. songs took the top ranking for the 'heartbreak song' category. The 1st place is held by Arata's Sakura to Tomo ni Kimi dake wo., the 2nd place goes to Kai's Sayonara Yume Hanabi, and the 3rd place is taken by Hajime and Haru's Koi Wasuregusa. It became a popular joke, where they form a fake sentai group called Heartbreak Trio.
- The joke is repeated now and then, like during April Fools at 2015 and 2016, and even during Tsukiuta. Stage performances.
- He doesn't like heavy make-ups or getting decorated gaudily, but due to his beautiful looks, it seems he's more often get that sort of job.
- Along with Arata, Rui, Kai, Yoru, and Shun, he's classified as the boke type. He seems to be unaware that he's a boke though.
- To be specific, Tsukino Mikoto seemed to classify him as the "boke with pure mind and common sense". Murase Dai from SolidS also belong to the same type.
- He likes sleeping and doesn't wake up easily, so he often becomes the target of Haru and Shun's prank if he takes a nap in the common room.
- He stands out even in the crowd, so usually he need to lightly disguise himself with glasses and hats when going out.
- He was questioned by a fan whether he doesn't like older people because said fan mistook the statement about him having soft spots on younger people. Explaining that he likes spoiling the younger ones, he also added that he'd like to get spoiled if he's with someone older than him.
- He has the look of his mother.