The characters are shallow and uninteresting, and the plot is likewise basic. So far there are three characters, with a fourth one alluded to. All members of the library committee at their high school. Oomori is the big-breasted kouhai who finds her senpai, Yuu, to be cute and she has a crush on him. She would like Yuu to look at her with a perverted gaze and to enjoy when she accidentally bumps into him with her big breasts. She says things like "If it's you, it's okay." But he seems largely disinterested, only getting flustered enough to correct her mistaken assumptions and to try to get free when she pulls his face into her bosom. Yuu has a complex about how short he is, and would rather be seen as cool than cute, which is probably why he acts all calm and collected (like a proper senpai should). But Oomori likes his cuteness. He's aware that she likes him (see ch. 9), but it's unclear whether or not he cares about her in the slightest. The third member is a black-haired girl who likes to tease other people. Oomori sees her as a love rival and is jealous of the close relationship she seems to have with Yuu.
I've made very few statements about each character's motives and thought processes. And even the statements I have made are largely based on deductions from their actions. And their actions seem more based on hitting cliches than actual character development, so trying to find any sort of depth to their personalities is probably an exercise in futility.
I can't imagine this manga getting any better as it goes on. It will probably continue to have slightly sexual/romantic encounters with no real feeling of drama or intrigue to them until it finally ends with the main couple getting together or something. In order to not come across as too stagnant, it'll probably introduce new characters periodically and go through the motions of having love triangles or struggles, but without any real sense of tension.
[Reviewed at chapter 12]