Itsuki Kisaragi is Ayato's twin brother. He was created from the same DNA as Ayato (Quon and Watari Shirow's), but instead of being taken care by Maya, Bahbem saw to it that he was raised as a wealthy child in his mansion, recieving high-end education along with Helena and Makoto, and taught how much superior he is to everybody else. Being raised on a very different enviornment, he developed very different tastes in clothing, food, and the like from Ayato. However, the two still shared a special connection since both apparently did the same painting. Throughout his childhood he became close friends with Makoto and even accompanies Makoto when he tries to escape the mansion with the living rock that was living under the mansion. Neither him nor Makoto never really got along with Helena since she often abused her position as Bahbem's neice.
As Itsuki got older he attended college, this is where he met Haruka and the two became lovers. Considering that Itsuki is Ayato's twin, it wouldn't be too far from the truth if we say that Haruka only hooked up with him because he looked so much like Ayato. Unfortunately for Itsuki, Haruka was still in love with Ayato who was stuck inside Tokyo Jupiter. The relationship doesn't work out.
During his time in college, Itsuki was entrusted with Quon's care who at this time has gotten over her coma, though she still needed special medical attention, an area Itsuki excelled at. (Most likely, Itsuki was trained and educated to take care of Quon). With time, Itsuki actually grew to love Quon as a real sister. He constantly worries for her safety.
At the end of the series Itsuki is killed by Sayoko in a crazy outburst. During his last dying breath Itsuki becomes delusional and sees Sayoko as Quon. Later we're shown a little tidbit of him holding hands with Sayoko, though most likely he held it because he thought Sayoko was Quon. In the apparent 'after-life' where Ayato, Haruka, Maya, and Quon were working out some inner turmoil we see that Itsuki approaches Quon through the phone and wishes to be with her. They later meet face to face in the classroom where Ayato and Haruka first kissed and exchanged a few words. We are sort of hinted that Itsuki will be with Quon after the tuning... but we're not told clearly how.