It's the book series called I"s
by Masakazu Katsura, and no that's not a miss-spelling either.
It's in the form of a graphic novel and over 140 chapters in length. Very finely illustrated to boot!
It's about a teenager and his 4 girlfriends that spans over 4 years time. From his freshman year of highschool to a year after graduating. It's a romantic comedy, but focuses far more on the relationships between the main characters then comedy. Which is spread lightly through the book series, pretty sublte humor too. It's so skillfully written that nearly every character that has dialogue(besides the main 5) becomes a semi-main character and has impact on the other characters throughout the series. The ALL of the main characters, and not just some, 'grow' as the series progresses. And all show great realism and could easily be pulled out of the book and set into real life with little problem.
It's written by japanese writer, so some of the concepts within maybe at first a little confusing but once you see their relavance that are easily understood. The other problem it that it doesn't have a buyable english translation. Even though it's been out for over 5years!
But it can be downloaded as a fansub from: http://isscans.cjb.net/
NOTE: getting the missing sections are easy and I highly suggest that you find them instead of passing over them, because a vital key main character is introduced who has alot of impact on the series itself.
is for the more mature reader do to it's situational content.