Ichika Nakano comments that the Nakano Quintuplets used to get along in the past  as they used to go to the bath together as five. Yotsuba Nakano remembers that Ichika was "the boss of the pack", although Ichika often bullied her. During their childhood, when the Nakano Quintuplets got sick, Rena Nakano (the Nakano Quintuplets' mother) always held their hands. Itsuki Nakano notices that the sisters' preferences are different from that of their past selves. Nino Nakano laments that the sisters used to be the same, but slowly change in the past 5 years. The current Nakano Quintuplets seem distant and not knowing of the others' state/situation.
This can be reinforced by the duo Nino-Itsuki and Miku Nakano-Ichika that are closer with each other, and seem to be doing activities together more often than with the other sisters. All five of them are quick to reconcile should they be ever at odds with one another. However, lately, they're rivals with each others due to competing for Fuutarou's affection, and Yotsuba seems to notice the bad mood around them.
The the Nakano Quintuplets love Rena Nakano (the Nakano Quintuplets' mother) dearly  where to this day, they still follow the Gotoubun Principles that was passed down from their mother. The Nakano Quintuplets, especially Nino Nakano, are eager to watch the fireworks because they used to watch it with their mother. This event is noted to be very important for the sisters because they value memories highly and  they are hinted to have a bad impression towards their biological father.
To Maruo Nakano, Nino seems to be wary of him. He is the reason why she rejects Fuutarou Uesugi's presence early on in the story. She also dislikes how Maruo calls the sisters with the "-kun" suffix. Despite the distant attitude of his daughters, Maruo is tolerant of their antics and even fakes holiday tickets so that they can meet their (maternal) Grandfather.
Early on in the story, most of the Nakano Quintuplets reject Fuutarou Uesugi due to various reasons, such as pessimism towards their learning capabilities, being protective, expecting a better tutor, or just because of a quarrel with him. Over time, due to Fuutarou's hard work, they began to accept and appreciate him one by one , where some of the sisters even began to develop romantic feelings for him and  Itsuki Nakano says that Fuutarou already brought a change to the Nakano Quintuplets. 
Raiha Uesugi is acquainted with all of the Nakano Quintuplets, but she is closer with Yotsuba Nakano and Itsuki. Raiha plays with Yotsuba during the Fireworks Festival, hangs out with Itsuki during the Labor Thanksgiving Day and later goes shopping together with them.
The Nakano Quintuplets are popular at school mainly because of their beautiful looks. At least three male students already express their adoration to Ichika Nakano . Nino Nakano has two female friends she usually hangs out with. Miku Nakano, despite being seemingly silent and reserved, has a female friend who calls her Miku-chan (instead of the more formal Nakano-san). Yotsuba Nakano has a lot of friends, although most of them are her friends because they need her help. She is the only Nakano Quintuplets to actively participate in various activities, such as helping the Basketball Club, Track Club, helping teachers, and offering to be the Class Representative.
Ebata is the secretary of Maruo Nakano where he personally took care of the Nakano Quintuplets when they were kids.  He has a good relationship with the sisters as he is willingly helps them for a personal matter. Itsuki Nakano is acquaintance with Shimoda, Rena Nakano's (the Nakano Quintuplets' mother) former student and she later works with her for a part-time job. Nino Nakano is working under Manager at a cakeshop named Revival and he praises her cooking skills on her first day.