Well, the English scans of the manga adaptation are already up to chapter 16 at this point, and Ai is not yet there. But it does kinda feel like time to introduce a new character soon after a few chapters or so.
I have not read the LN, so I do not actually know if she is going to be introduced soon, though.
Assuming this is coming out in 2013, I wonder which season we can expect it to air?