WARNING: The following contains spoilers for Episode 12 of Komi Can't Communicate, "It's Just the Culture Festival," now streaming on Netflix.
Yamai Ren has had a huge schoolgirl crush on Komi Shoko since the first day of school. She loves Komi so much, she even kidnapped her best friend Tadano Hitohito one day and hid him inside her bedroom closet just to be able to sit next to Komi in class. She even threatened to kill Tadano for being close to Komi as an "unworthy" person, and she threatened his life again if he made any sounds inside her closet while Komi and Najimi were visiting. This did not end well.
Luckily for Yamai, Komi was very forgiving of her yandere behavior, albeit most likely due to the latter's lack of awareness than a real fear of yandere individuals. While Yamai only succeeded in obtaining Komi's friendship, this didn't stop her from wanting to pursue a romantic relationship with her. Their friendship has similarly not stopped Yamai from competing against Komi's other friends and suitors for her affection. This has been the pattern of Komi and Yamai's interactions throughout Komi Can't Communicate Season 1, so it was only a matter of time before Yamai would try to take things to a whole new level.
During the Itan Private High School culture club in Episode 12, Najmi got the "bright idea" to exploit Komi's popularity with the students as part of a profiteering tactic to draw more customers into their class's maid café. This didn't sit well with Yamai, who found herself "protecting" Komi from being taken advantage of by opportunistic students, especially during the "be slapped by a maid" event. Despite her best efforts to preserve "Komi's integrity," however, Yamai also couldn't help but take advantage of a festival event to try and progress her own relationship with Komi.
One of the events being held at the festival was "rooftop confessions," in which students would stand on the school rooftop in front of a large crowd and confess their love to their crushes. The event also risked humiliation if the person making the love confession was rejected on the spot by their crush -- as the guy who professed his love for his classmate Watanabe quickly found out. Still, Yamai couldn't help but take the plunge. Without any hesitation on her part, Yamai very loudly and clearly professed her love for Komi, asking her to marry in addition to dating her. This left the crowd -- including Komi -- speechless, causing Yamai to interpret the silence as symptomatic of Komi's shyness, which she finds endearing.
Despite Yamai's confession leaving every student stunned to the point of focusing their attention on Komi, the latter was surprisingly calm throughout the whole thing. She didn't even tremble from anxiety despite her strong aversion to being stared at by large groups of people. This could mean one of two things: either Komi was still processing Yamai's confession internally or she took it for what it was and treated it as normal behavior for Yamai. After all, she's not oblivious to Yamai's feelings for her, and nor does she find her behavior bothersome since she's not threatening the lives of her friends anymore.
After the festival ended and Najimi invited their group of friends to karaoke, both Yamai and Komi continued acting like normal. At the karaoke bar, Yamai once again professed her love for Komi by dedicating a love song to her. Unfortunately, Komi was unable to fully appreciate Yamai's singing because she was feeling too anxious about the thought of doing so herself in front of their group of friends. While Komi is not able to reciprocate Yamai's feelings due to her crush on Tadano, she continues to value having her as a friend. More than she wants to date, Komi wants people to share experiences with in order to compensate for a lonely childhood.