Keigo - Uses this; it, and her Ojou status, are the reason why most of the other kids avoid her at school, making her lonely.
Neat Freak - Once when the protagonist asks her how things have been going at home lately, she talks excitedly about this amazing new sponge her mum picked up recently that can clean up anything, but that she's now banned from using it because she overdid it and started polishing things so hard they became discoloured.
Not a Date - Manaka and the protagonist go on a pretend date so she can see what they're like (so she can better chaperone a young girl she's tutoring), but she is insistent that it isn't an actual date. At the end, she blushingly tells him that it might have actually been a date, after all...
No, You Hang Up First - Manaka tells the protagonist she doesn't want to hang up on him, so he tells her to hang up first. She says she really doesn't want to... but then does anyway. The protagonist is...surprised.
Ojou - So sheltered that she has apparently never even been to a fast food restaurant.
Peerless Love Interest: Manaka is seen as this by the other students due to her great grades, beautiful looks, amazing tennis skills, high-standing family, and overly-polite behaviour. For this reason, no-one really tries to get close to her, causing her some deal of angst and social awkwardness as she isn't sure how to make genuine friends. One background character even says at one point that it's like she lives in a totally different world to everyone else.
Big Brother Instinct - Rinko worries this is why Wataru (the protagonist) is treating her so nicely, since she is one grade below him.
Gamer Chick - After the player and Rinko finish working at the school library, she may take the player to the arcade, where she will reveal her skill in fighting games. After you see her genuinely having a good time for the first time, she comments that you should practice more, since she would expect her playing partner to be better.
She even nearly gets into a fight with a well-muscled youth in a wifebeater due to his inability to accept his loss in a fighting game to a girl. As the situation escalated, Rinko apparently threw the first kick. When she sees the player nearby, she claims that he is her boyfriend and asks the player to "destroy the punk." Your seething opponent is stepping up to the challenge when the player grabs Rinko by the hand and employs the classic Nike defense.
Differs in the manga version, in that the Wataru voluntarily (and nervously) steps up to the plate and claims to be her boyfriend, just seconds before she can finish her proclamation that He Is Not My Boyfriend.
Going to the Store - Rinko bumps into the player at a local convenience store fairly often considering the store is local to the player's area and not hers. Just a coincidence, and none of the player's business, of course.
Later in the game, if the player and Rinko grow close enough, the player will receive a text in the middle of the night from Rinko about a sale on his favorite late-night snack. Sure enough, when the player goes out to check, there she is, waiting for him in the store with a can of said snack. She had walked all the way there to talk to him, but when he wasn't there like he usually was, she didn't want to run the risk of waking him up by calling him, so she texted him instead. Aww.
Headphones Equal Isolation - She can be seen sporting a pair of headphones fairly often, even at school. In fact, the first time you hear about Rinko, it's when a female student tells her friend how Rinko ignored her greeting and doesn't seem to fit in with the other students.
Joshikousei - Played with Rinko; she seemingly randomly chooses not to wear her uniform in school and to wear her uniform outside of school. You later learn that the latter is partly due to Rinko avoiding her house because of her discomfort with her father remarrying.
Older than They Look - Used to keep her hair long in middle school but got frustrated with people mistaking her for an elementary school student.
Small Reference Pools- Subverted. A Japanese dating sim references The Catcher in the Rye. Specifically, Rinko Kobayakawa quotes from the book at one point, referring to the relationship between Holden Caulfield and his sister Phoebe.
It is noteworthy that Kobayakawa herself initally resembles Holden Caulfield in her contemptuous attitude towards society and those around her. Although, she mellows out quite a bit after meeting the protagonist.