Forty is Hitoshi's A.I. Program No. 40 (Nanbā Fōti "Number Forty"), and the first he brought to life on purpose (sort of). Created using both C and Assembly Language, he/she is more powerful than Saati and Toeni combined. Saati and Toeni sneaked a before-it-was-ready look at this new A.I., agreed on an age, twelve, and then argued about the gender and personality and pressed too many buttons at once. The end result was an A.I. that had two modes: a male personality and a female personality within a female body. When Forty-kun, the boy, said "girl", he became Forty-chan, the girl; when Forty-chan said "boy", she became Forty-kun. While each has presumably heard of the other's existence there are only two instances shown in which Forty-chan has some consciousness of Forty-kun; mostly each is blithely unaware of the other.
Forty-kun describes himself as a hard-line conservative who is not interested in Hitoshi's porn-based hobbies, an anal-retentive neat-freak, a self-righteous good boy, with a taste for the nicer things in life - Saati's personality choice. Forty-chan describes herself as sensual party girl with a taste for the wild side of things, even going so far as to seduce Hitoshi by using a Hyper Program Option to give herself a more mature body - Toeni's personality choice. Saati wanted a nice little sister, and Toeni wanted a naughty little brother; they both got exactly what they wanted in one form.
Forty-kun got it into his head that he wanted to go to school, so he was enrolled in class 1-C in what looks like Jr. High and, convenient for the manga-ka, located in the same building as Hitoshi and Saati's High School.