Yoru is a young male with black hair and blue eyes. He has narrow and sloped shoulders which makes him fit ladies' size clothes better than men's size clothes.
Yoru is earnest, sincere, and very mature. He does not like doing things rashly, and always prepares everything beforehand. His high level of femininity - due to the fact that his personality comes from his mother, as well as his high skill in cooking - has brought him the position of "Procellarum's Mother". While he may appear docile and calm, according to Haduki You, he can also be very stubborn during certain situations, quite similar to his grandfather. Yoru also possess a strange sense of humor, and is easily struck by bad punchlines.
Despite being an idol, he considers himself to be quite plain, compared to the others. He is also shy, often choosing to avoid photoshoot jobs and the like, but is gradually growing used to them.
Yoru is an only child. His father's job has not been revealed. His mother works at a foreign company. Yoru often helped his mother with cooking because this was how they bonded, which resulted in his high cooking skill.
He and You were scouted together during a fieldtrip to Tokyo.
Yoru and You are childhood friends, and they get along very well. You greatly enjoys Yoru's cooking, and has claimed Yoru to be the best when it comes to cooking.
Whenever they have misunderstandings or arguments, most of time, while You is the one at fault, although Yoru is the one who apologizes first. You tends to initially apologize sharply, but once away from people's gazes, he apologizes earnestly.
Due to Yoru having a negative way of thinking, he usually asks for Shun to hear him out. Shun has a positive way of thinking which helps Yoru with his problems. 
Together with Aoi, they become a mother-combi in the dorm since they have similar feminine hobbies such as cooking. When the members in the dorm have a day break, they cook meals together. 
Yoru's surname, Nagatsuki (長月), is the archaic name for the ninth month of the lunar calendar; poetically, September. When broken into the individual characters, Nagatsuki is made is up of two characters: the first meaning "long" and the second, "moon" or "month". One theory states that nagatsuki is the abbreviated form of yonagatsuki (夜長月), which translates to "the month with long nights". Since middle ages, people believed that the ninth month of the Chinese lunar calendar signaled the coming of longer nights, in contrast to the short nights experienced during the summer.
His given name, Yoru (夜), was given to him because he had been born on a beautiful, moonlit night.
When his given name and surname are put together in Western order, they form the possible origin of the word nagatsuki, yonagatsuki (夜長月).
- His eyesight is 0.6 and is the one with the weakest sight strength among the males
- However, he doesn't wear glasses often because he gets headaches when wearing them for a long period of time. He also doesn't use contacts because they hurt as well.