Rui is a lean, rather short young male with greyish-green hair, with several locks reaching past his shoulder and pale olive-colored eyes. He has a scar on his body that resulted from an incident when he was five years old and constantly has a band-aid on the left side of his neck and another on his left chest area.
He has a stiff body, sensitive skin, low blood pressure, noise-sensitive, among other things. However, he started building up his stamina, although it remains at a level below average.
Rui comes off as innocent and naive, to the point that Shimotsuki Shun has (somewhat negatively) influenced his way of thinking. However, he can also do rather unexpected things while maintaining a straight face. He possesses a unique worldview, which makes him nonreactive to things such as scary stories.
Rui possesses a natural talent for music, particularly the piano, and has participated in national and international competitions from childhood. However, following a discord with his older brother, Rei, who did not possess the same musical talent as Rui, as well as disagreements with his parents involving his musical career as a pianist, Rui has run away from home, was picked up by Fuduki Kai and had been living with him until they were scouted. He was scouted due to Kai (who had been scouted while helping out with some people moving into the talent production's dorms) showing the talent company a picture of Rui, who had just woken up, looking out-of-focus, and said the company encouraged Kai to bring Rui along to become an idol.
Kannaduki Iku and Rui had a lot of distance at the start, but Rui started to open up to him, which is the result of Rui calling Iku for "Ikkun" . He also is the only one that Rui calls by nickname. Rui calls other members that live in the dorm by their names only. 
Yuno and Rui seems to be on good terms, as Rui lets Yuno play with his cat Yamato. Yuno is taller and stronger than Rui, proved that once she was able to carry him bridal style, which apparently made Rui a little mortified.
Kai took in Rui after the latter ran away from home, even Yamato, who Rui took with him. Kai, quite used to taking care of those younger than him, treats Rui as if he were his son, or at least, a younger brother of his.
Because Rui is naïve and innocent, Shun likes to teach him a lot of weird things. Rui doesn’t like how Shun looks happy whenever he missed when he plays bowling.
Because his older brother was a classmate of Mutsuki Hajime, he had been told about Hajime, and at the time before he met with Hajime, he had a one-sided acquaintance with him.
Rui considers him as a mother of Procellarum since Yoru is always taking care of Procellarum members. Rui likes pudding the one Yoru made for him too.
Kakeru worried about Rui when Rui stopped to take a rest in an errand for Kai. He asked if Rui feels fine. Later on, when they have lived in the same dorm, their relationship develops pretty well.
- His name reflects many of his traits:
- His surname, Minaduki (水無月, Minadzuki), is the name of the sixth month of the lunar calendar.
- His given name, Rui (涙), literally translates to either "tears" or "sympathy".
- The kanji of his name can also be read as "shizuku", which translates to "drop of water". Thus his name is an allusion to tsuyu, the rainy season usually occurring from early-June to mid-July.
- Similarly, his and Terase Yuno's official fanclub also reflects their position as June representatives:
- Their official fanclub name, pluvia, is derived from Latin has the meaning of "rain".
- Their fanclub motif is that of a cat and droplets of water. The cat represents Rui's pet cat, Yamato, while the droplets of water represents the rainy season that often occurs in June.
- Their crest is that of a hydrangea, which blooms on rainy season and thus often affiliated with June.
- His eyesight is 1.0.
- He is said to be mother-like and the most cat-like among the 12 idols.
- He is left-handed. According to Iku, Rui's handwriting is unique.
- Sometimes, he talks to Yamato.
- He is weak towards both heat and cold.
- He can't eat hot food, just like cats.
- His favorite foods are pudding, omurice and egg sandwich.
- His favorite pudding is the one Yoru made for him.
- He has an older brother named Rei
- After his confrontation with his brother and parents, he ran away from home and met Fuduki Kai.
- His mother is a pianist and his father is a conductor.
- He has some scars on his body.
- After one year has passed since Rui entered the dormitory, he has reconciled with his older brother.
- He is not good at building relationships with other people. When he just entered the dorm, he put quite a distance with other members.
- During the time of debut, he stated that he actually doesn’t want to be an idol. But after being able to get closer to other members, he changed his mind.
- According to him, every Procellarum member has their own voice tone and he managed to see through their personality with it.
- About Shun: “It’s like the music itself, comfortably flowing. It’s like he’s laughing while singing about fun things, so I don’t feel any bad feeling from it.”
- About Kai: “It’s like a calm big ocean, listening to it makes me feel at ease.”
- About You: “It portrays freedom. It’s uneven and fierce, but is actually really kind”
- About Yoru: “It’s full of lucidity and gentleness. It feels like you’re wrapped on it.”
- About Iku: “It’s bright and full of spirit. It has the power to keep on being positive and to move forward.
- He is really naïve and innocent. That’s why Shun likes to teach him many weird things (And sometimes Arata does too)
- He doesn’t attend school
- He is really bad at drawing, but he doesn’t seem to notice it
- His body is terribly stiff
- He claimed that he’s not frail, but in reality he has no physical strength. Iku often tells him, “It’s okay even if it’s just a light exercise, so please do some exercise!” but he almost always refuses it.
- He is a terrible dancer (As noted by Arata)
- He is into bowling lately after Shun, You and Koi took him to play with them. He doesn’t like how Shun looks happy whenever he missed, so he practiced a lot to the point that he could typically hit 2 or 3 pins now
- Lately he starts composing songs
- He still believes in Santa Claus
- Until now, most members even Iku doesn’t know his family circumstances. Iku said he wanted to hear it directly from Rui and will wait until he’s willing to talk to him. 
- According to Terase Yuno, Rui's easily misunderstood.  He doesn't show his emotions much, and he also doesn't talk very often, so people would think he is cold and not friendly.