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Your very existence has already begun to stir a change in the five of us.
|― Itsuki Nakano to Fuutarou|
Itsuki Nakano (
AppearanceItsuki is a serious looking girl with medium-length hair and an expressive ahoge. She has reddish-orange hair color, ranging from light pink to salmon pink in Volume 1, and deep orange to orient red in Volume 4. Her notable accessories are a pair of star-shaped hairpins, which Fuutarou Uesugi called unfashionable at one point, near both of her eyes. Like her sisters, she has dark blue eyes, an average height, and a well-endowed figure. In class, or when she does something involving studying, she wears glasses.
Itsuki is a serious and earnest girl  who usually wakes up early compared to her sisters.. She is also well-mannered: She cares a lot about eating manners, has never been late or missed a class , and always speak in polite language.
Itsuki is a girl who bears a grudge, shown when she rejects Fuutarou Uesugi's tutoring partly because of their previous quarrel. She is also someone who can't be honest with herself and has difficulties being more straightforward. Both of these qualities often make her struggle on her own. Nino Nakano comments that Itsuki is too straight-laced. Itsuki is too precise in her timing during cooking.
Itsuki has a huge appetite and is mostly seen eating something throughout the series. It appears that she doesn't mind treating her sisters unfairly when it comes to food-related stuff. Itsuki proposes to share their cakes and Ichika Nakano suspects her real goal is to sample all of the flavors. Itsuki worries about Fuutarou, but it's actually because she's hungry.
According to Fuutarou, each Nakano Quintuplet has their own characteristics regarding their sheets. In Itsuki's case, she would rather use an eraser than scribble on them
According to Ichika, Fuutarou has to be firm to deal with Nino and be kind to deal with Itsuki.
Itsuki is commended as a diligent girl, and she does things very precisely. She is also perceptive and has a good sense of detail. Despite that, she has an awful sense of direction. She also has poor eyesight and is often seen wearing glasses. Itsuki has good observation and deduction skills. Among the sister, Itsuki ranks on the higher part of the five, with her strongest subject being Science.
- Main article: Itsuki Nakano/Relationships
- "In the end, we can't know what he feels, what he thinks, unless we ask him personally"
- "Aren't you going to set a good example? We're counting on you"
- "How does it feel to receive words of gratitude from someone you taught? For me. I want to treasure the feelings from that time"
- "The numerous days we spent studying for exams, fireworks festival, field trip, new year celebration, we spent all that time together. Doesn't that already make us... friends?"' 
- Itsuki is the only girl whose name is a portmanteau as the name Itsuki means "five" (五) (itsu) and "moon" (月) (tsuki).
- It can also mean "tree" (樹).
- Part of the name Itsu means "mercy" (慈), "flee, escape, break loose" (逸) or "graceful" (淑).
- Itsuki is labeled as the "first girl" of the series.
- Itsuki is seen wearing glasses when she's about to do a test (Chapter 2), suggesting she has hyperopia (far-sightedness). In Chapter 3 with Fuutarou Uesugi, he remarks that Itsuki can't see far away, suggesting she has myopia (near-sightedness). Despite sharing eye problems with Nino Nakano, only Itsuki wears glasses.
- Itsuki believes in fate and charms.
- Itsuki's notable accessories are two stars on the front of her hair, representing number 5, which is a part of her name and the order of her birth.
- Itsuki won 3rd place on the Japanese popularity poll with 1235 votes.
- Itsuki's food critic pseudonym, May (M·A·Y?) literally means May in English, as in the fifth month of the year. The Kanji of Itsuki (五月?) is also used for "May" in Japanese.
- Itsuki is the second Nakano Quintuplet to get her picture taken alongside Fuutarou. However, she is the first Nakano Quintuplet to have a picture with Fuutarou taken properly.
- Itsuki is the first Nakano Quintuplet to receive a "kabedon" from Fuutarou in Chapter 1.
- Itsuki is the first Nakano Quintuplet to be seen crying. She was implied to cry at the Fireworks Festival Arc.
- Itsuki is the first Nakano Quintuplet to hold Fuutarou's hand at the start of the bonfire.
- Itsuki is the third Nakano Quintuplet to have unintentionally or indirectly confessed to Fuutarou.
- Itsuki is the third Nakano Quintuplet to be seen half-naked by Fuutarou (in Chapter 63),