The “Tenshi-tachi no Gogo” series is the ancestor of the modern VN, first released by JAST in 1985. She is the heroine of the fourth in numbering, actually the seventh in the same series.
By the way, the protagonist has a fiancée, so if you do it with Yuuko, it's game over for you in all routes.
https://vndb.org/v8772

nonomura_yuuko, 1girl, purple eyes, blonde hair, long hair, bangs, hair between eyes, sidelocks, hairband, red hairband, school uniform, serafuku, long sleeves, sailor collar, purple sailor collar, neckerchief, yellow neckerchief, bow, pleated skirt, purple skirt, white legwear, loafers