The title you provided, Toshoshitsu no Kanojo: Seiso na Kimi ga Ochiru made (roughly translating to "My Girlfriend in the Library: Until You, the Wholesome One, Fall"), refers to a specific sub-genre of visual novels and eroge—specifically the NTR (Netorare) or "corruption" arc.
Here is a short narrative piece exploring the themes and atmosphere typically associated with this title, focusing on the contrast between the setting and the psychological descent. toshoshitsu no kanojo seiso na kimi ga ochiru m free
You play as a quiet library-goer who notices a seemingly innocent, “seiso” (pure/clean) female student always studying in the same corner. Through small choices, you slowly peel back her reserved exterior. The premise isn’t groundbreaking, but the free version does a decent job establishing a slow-burn tension in a cozy, nostalgic library atmosphere. The title you provided, Toshoshitsu no Kanojo: Seiso