Itsuki Nakano (
Itsuki is a serious looking girl with medium-length hair and an expressive ahoge. She has reddish-orange hair colour, 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 near both of her eyes, which Fuutarou Uesugi called unfashionable at one point. Like her sisters, she has dark blue eyes, an average height, and a well-endowed figure. In class, or when she does activities that involve academics, she wears glasses.
Itsuki is a serious and earnest girl  who usually wakes up early compared to her sisters. She is well-mannered; never being late or missed a class, and always speaks formally. In the Japanese script, she almost exclusively speaks with a keigo speech pattern. Nino, her sister, comments that Itsuki is too straight-laced.
Itsuki is a girl who bears grudges, as shown when she rejects Fuutarou Uesugi's tutoring partly because of a previous quarrel they had. 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. She is easily agitated, be it about unfavourable situations, scary things or her stubborn self.
She has a huge appetite and is frequently seen eating something or being hungry throughout the series. It appears that she doesn't mind treating her sisters unfairly when it comes to food-related topics. This shows where during one scenario when she proposed to her sisters to share their cakes; suspected to sample all the flavours by Ichika.
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
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. Her poor sense of direction is first mentioned, when she became sensitive to the cold so she sneaks into Nino's futon. She also has poor eyesight and is often seen wearing glasses. She has good observation and deduction skills. On exams, Itsuki generally scores high compared to her sisters, 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 (a word blending the sounds and combining the meanings of two others, for example motel (from ‘motor’ and ‘hotel’) or brunch (from ‘breakfast’ and ‘lunch’)) as the name Itsuki means "five" (五) (itsu) and "moon" (月) (tsuki).
- Unlike her sisters, Itsuki was born on May 6 at 12:00 AM, this is confirmed by the manga in volume 14 since, according to the character file, the following is indicated: "She was born when the date changed, but she tries to prevent someone from investigating this fact too deeply "...
- 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.
- She and Nino seem to be the most similar to each other among the quintuplets (both of them do sit-ups in the morning, have long hair, poor eyesight, a fear of needles, and a tsundere-like personality, being initially harsh to Fuutarou). Both of them are also close to each other and often seen hanging out, and Nino is very vocal and protective regarding Itsuki.
- 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 habitually speaks honourifically in Japanese. She uses a speech form, keigo, which is the polite form of speech. She addresses Fuutarou by his surname referring to him as "Uesugi-kun", and ends her sentences in "desu" (です) and "masu" (ます), or variations of the phrases, which are speech particles added as a formality in sentences.