An original character by Igarashi Kyouhei generally cいとこのお姉ちゃん(妹) or いとこのお姉ちゃんの妹 in the commentaries of the works she appears in. Despite the current tag name, she is still older than the protagonist, just younger than her sister.
She has Brown Eyes, Large Breasts, a mole under her eye, and medium lengthed Purple Hair (sometimes seen as black) stylized in Twin Drills, Sidelocks, and Bangs.