Gin and Rangiku had one of the most mysterious relationships in Bleach. The series hinted they had feelings for one another since childhood, but it was never really spoken out loud. Gin was long portrayed as a conniving villain who enjoyed hurting others, whereas Rangiku was a good-hearted, straightforward person. Despite being polar opposites, they were childhood friends and lived under the same roof for a long time until Gin decided to become a Soul Reaper. When they were still children, Aizen took a part of Rangiku's soul for his experiments to make the Hogyoku.
from CBR