Instruction: Your job is to summarize this article below, while focusing on that topic "What is the plot of Onimai?"
Input: Onimai: I\'m Now Your Sister! (Japanese: お兄ちゃんはおしまい!, NAME_1: Onii-chan wa Oshimai!, lit. \'Big Brother is NAME_2 For!\')[a] is a Japanese manga series written and illustrated by NAME_3. The series has been serialized online since 2017; An anime television series adaptation by NAME_4 aired from January to March 2023, receiving positive reception from critics and gaining high ratings, although it received a mixed reception from English language critics.  Plot[edit]  The series follows NAME_5, an otaku and NEET who lives alone with his scientist younger sister NAME_6. One day he wakes up to find himself transformed into a young girl, a result of one of NAME_6\'s experiments. NAME_7 must now learn to become comfortable in his new body whilst living life as a middle school girl.  Characters[edit]  NAME_5 (緒山 真尋, NAME_8)[b] Voiced by: NAME_9 (drama CD, anime)[2] The protagonist of the series, NAME_7 was previously an otaku man living with his younger sister NAME_6 until he was turned into a girl as a result of one of her experiments. NAME_7 ends up transferring to a middle school. NAME_10 (緒山 みはり, NAME_11 NAME_6) Voiced by: NAME_12 (drama CD, anime)[2] NAME_7\'s younger sister and a scientist. She turned NAME_7 into a girl as a result of one of her experiments. Because of her high intelligence, she skipped several grades and is now in university. NAME_13