The character Yamato Nagamori (永森 やまと) from the PS2 visual novel Lucky Star: Ryōō Gakuen Ōtōsai (らき☆すた ~陵桜学園 桜藤祭). She is a mysterious transfer student who transfers along with the protagonist, and only appears in that game.
yamato_nagamori _lucky_star_ill_v1.safetensors