The plot in Oreimo Portable acts as a spin-off to the light novels, which explores the relationship between Kousaka's siblings Kirino and Kyousuke. The story begins the year after Kirino's infamous "life consultation" with her older brother, who accidentally discovers her darkest secret: that she is an avid fan and collector of anime and eroge involving older brother-little sister dynamics. After Kirino returns from her trip to the United States, she and Kyousuke continue on the usual antics, with their friends Saori Bajeena and Kuroneko tagging along. At this time, Kuroneko is starting to become close to Kyousuke during Kirino's absence, and their relationship is used by Kuroneko to tease Kirino, who is her rival to Kyousuke, and — surprisingly — her best friend. The player assumes the role of Kyousuke Kousaka, whose life as a mediocre person is changed by his 'life consultation" session with the little sister whom he has learned to ignore. The story later develops into Kyousuke's experiences with the different girls who have interacted with him, including Kirino, her best friend Ayase Aragaki, and her classmate Kanako Kurusu.