Saeko Sasaki is a primary character in the series 'Ikenai Oneesan'. She is portrayed as an attractive and charismatic woman who often finds herself in humorous and compromising situations. Saeko's role in the series involves being the 'oneesan' or big sister figure, who is admired and sometimes envied by others for her confidence and allure. Despite her outward appearance, she is also depicted as having a caring and protective nature, especially towards her younger siblings and friends. Her character adds depth and humor to the storyline, making her a fan favorite.