In Karte 12, Yamada was in the school infirmary because, in her words, her stomach hurts. Ichikawa assumed that it's because she ate something bad, but it wasn't the case since she ate candy while she was looking for pain medication to give to Ichikawa for his headache. As Norio Sakurai later tweeted during the airing of the anime, it's actually because she had menstrual cramps (which is also why she had that pain medication to begin with, along with explaining her short period of silence before she answered.)
Ichikawa anonymously left tissues for Yamada at the library in order to cheer her up after she was injured, along with a note written on one of the tissues saying that it was for her to use. Aside from the two using the library the most, Yamada previously commended Ichikawa for his handwriting during the setup for the school cultural festival, so she knows that he left those tissues.note This only applies in the anime; in the manga Yamada commended him for his art because of the drawing he gave her in Karte 3.
It would also explain why she actively seeks him out from that point forward.
Normally Ichikawa is the narrator of the story, but there is one instance that somewhat pulls The All-Concealing "I", and one that only exists in the original Japanese manga release. In Karte 30, if one looks closely, the narration boxes use two different fonts; the first three boxes used a brush-stroke font, while the rest used a modern font. It can be deduced that there are two narrators in the chapter and who they are; the brush-stroke font represents Yamada, while the modern font represents Ichikawa. But then there's the scene in the library where Ichikawa was holding Yamada's hand in order to stop her from eating chocolate and getting caught by the library teacher; when the narration says that "好きだ", it's logical to assume that this is Ichikawa reaffirming that he likes Yamada. But it uses the brush-stroke font, meaning that the narration is actually from Yamada, realizing that she likes, or at least how much she likes Ichikawa.
The anime adaptation of Karte 39 has Ichikawa fall unconscious face-first into Yamada's chest instead of her shoulder like in the manga. A strange choice, but in the same episode includes Yamada making a chart of where-and-where-not to touch her, which of course includes her chest (As Moeko demonstrated). Her still not trying to move Ichi could be interpreted as a weird, adaptational instance of If It's You, It's Okay.
Yamada left a note after she took care of Ichikawa (who passed out from his fever). The note in question is folded in a specific manner and had unusual creases; if followed it's an unfinished origami heart. Yamada tried to make it but chickened out at the last minute.
In Karte 51, Yamada mildly calls out Ichikawa for hanging up on her when his sister catches them on video chat. She was also nervously saying hello before he turned his phone off. She's still holding Ichikawa to his promise from the shopping center in Karte 47 to introduce her to Kana.
In the movie theater in Karte 78, Yamada wants to switch seats with Ichikawa so that she sits on his left side, likely because she wants to see his face since the right side is covered in his bangs.
Out of all the girls that get close/talk to Ichikawa, Yamada seems particularly jealous of Hara despite knowing she has a thing for Kanzaki and being in otherwise good terms with her. Why? One of the first major acts of kindness she discovered from Ichikawa was trying to pull a Zero-Approval Gambit to avert Hara getting hate from her classmates back in Karte 10 when they were blaming him for writing "Yamada" on one of the gravestones of the cemetery backdrop for the culture festival. So from her perspective, Ichikawa is not only open to defend Hara but willingly got in a position that could have gotten him hated by her for Hara's sake.
In Karte 121, Ichikawa is re-reading Kimi-Iro Octave to reflect about the role Yamada is gonna have in the drama and stars ranting about the Teacher/Student Romance finding it inappropriate which Nigorikawa agrees. At first glance it just seems like it’s just his conscience agreeing with himself, until you remember Nigorikawa is a character from those same manga, so he's actually stating the opinion of the character in-universe about the whole affair.
, it's actually because she had menstrual cramps (which is also why she had that pain medication to begin with, along with explaining her short period of silence before she answered.)