Its important to remember that Bleach is a shounen manga, meant for boys. Its an action/adventure title that focuses on battles.
However, Kubo Tite is a masterful writer. He manages incredible amounts of characterization through relationships, character actions, supporting character observations, context and symbolism while still delivering on the fight-heavy premise of the genre.
Ichigo and Rukia are the core of Bleach. Their meeting is what started it all, that one night when Destiny brought them together (read: Chapter 00a, The Sand, and 00b, The Rotator). Its repeated by characters and shown through actions that their bond is something special and unique. Even Kubo Tite has said that they are "more than friends, but not quite amorous" during several promotionals for the third Bleach movie - "Fade to Black". If Bleach were a shoujo manga, they would be the canon romance.