Rena has an obsession with all things cute. In what the others call her "Cute Mode" (かぁいい・Kāii), she often tries to take such things home in her Take Home (お持ち帰り・omochikaeri) Mode even if they do not belong to her. Such things include the "Kenta-kun" doll, Furude Rika, Hanyū, Satoko, and any of the club members in cute punishment game outfits. She spends much of her time "treasure hunting" at the town's trash heap in search of cute things which often appeal only to her. In the trash heap, Rena has a secret base consisting of an old van she has filled with things she likes to which she retreats when distressed. Nevertheless, according to Mion, while Rena might seem cute herself, people should be careful not to anger her as she can become quite scary. Keiichi discovers this in an infamous scene in the Sound Novels, manga, and anime where she suddenly screams "Lie (嘘だ・うそだ・usoda)!"

Rena's parents are fashion designers. While she was born in Hinamizawa, her family soon moved away to Ibaraki for the sake of her mother's advancing career, and she used her given first name "Reina (礼奈)." Her mother starts an affair with a co-worker. While Rena meets him and found him agreeable, she does not understand her mother's relationship with him until her mother asks her to come away with her to start a new family when she reveals that she is pregnant with the man's child. Rena refuses even though her mother threatens to never see her again. Rena's father is devastated, and Rena blames herself for not recognizing her mother's affair.

Whenever Oyashiro-sama is mentioned, Rena's personality changes such that she insists that he is real and becomes fiercely defensive. She truly believes in the god's existence and is thankful to him for many things. Rena also has an intense hatred for lies, often lashing out violently when being lied.

Rejecting her position as her mother's daughter, she begins to insist on being called "Rena" instead of "Reina." According to the Tsumihorobo-hen TIPS the removal of the "i" in her name was to "remove the icky things" (いやなこと・iyana koto), though later she realizes that "the 'incredible things' (いいこと・ii koto) also start with an 'i'." She remains very loyal to her father, and as revealed in Tsumihorobo-hen, she will go to violent lengths to protect him. She is very sensitive about the topic of Oyashiro-sama, who she believes cursed her for leaving Hinamizawa. She cuts herself trying to get rid of the "filthy blood" left in her by her mother that she imagines is filled with maggots.

Despite her silliness, Rena sometimes shows great intuition and responsibility, displaying a terrifyingly perceptive side of herself that can read people and situations to their very core at a glance. Rena is also aware of how often she goes into Kyute Mode and will use this to her advantage at times.

Rena shows hints of depression and sadness, given the circumstances surrounding her parents divorce. Sometimes she will say her own catchphrases to herself to cheer herself up. Rena also has a secret base in the junkyard formed out of a wrecked van that she will go to for peace and quiet. Rena feels a strong attachment to the junk she brings home, thinking of them as abandoned and needing a place to live.

Rena has short brunt orange hair in an angled cut with long sides and her fringe parts on the left of her forehead. Her eyes are lavender blue and droop at the ends. On her school days, Rena wears a fairly typical sailor fuku, complete with a yellow ribbon, and white socks with dark brown shoes. On her free days, she wears a white dress with a purple bow and sash and deep silt up the middle, complete with "her favorite" white cap. She also wears black thigh-high socks with brown knee-high boots. She wears a similar hairstyle with longer hair and has grown taller than she was in June, 1983.

Ohayou! Why are you taking so long-kana... kana~?