She and Will became friends early in the book. In the late part of stage 2 of initiations Will kissed Christina as they were walking near the train tracks, later followed by Christina kissing Will.
Will and Christina start dating just before the 3rd stage of initiation when Will punches Christina. Their relationship continues through the end of initiations where they celebrate becoming Dauntless members together. At the end of the book Tris is worried if she will ever see Christina again and is terrified to think what will happen when Christina finds Will's lifeless body in the streets.
When Christina found out that Tris had been the one who shot Will, she was very upset and did not speak to her for days. Eventually, she forgave Tris when she realized that there really was no other option and that he was under the simulation.
On AO3 the ship is the most written ship for Will and the Christina. It is also the fifth most written ship in the Divergent Series - Veronica Roth and Divergent all media Types tags, and the seventh most written in the Divergent (Movies) tag.