In chapter 56 a figure-head in the Witch communities mentions a prophecy.
In the days when light and dark leave the same basket,From there the tenth of this world shall begin,The child's fruit heals and the vessel sprouts,The light and dark will set foot in the east,And become the child's guiding hands,The child must choose a path,Will they become ten or stay as one?
The "Child mentioned in the above is Chinatsu, whereas the "light and dark" and "the child's guiding hands" refer to Akane and Mokoto since their mana types are Light and Dark repectively. "The light and dark will set foot in the east." refers to Makoto and Akane moving in with Chinatsu's family. Since Chinatsu is already learning magic from them the prophecy is already in motion.
Given how close the two have become, it would make sense that the shore rabbit will become Chinatsu's familiar when she becomes a full fledged witch.