You need to get Aburama for danzo to be unlocked. Same thing with mifune, you need to recruit hanzo.

Yep, just like Haine said .. Some ninjas needs to be unlocked by recruiting the prerequisite ninja of it .. With Danzo's case, you have to recruit Torune and Fu first before him to be available for you to redeem frags .. It is the same process for Gaara as he needs Kankuro and Temari to be recruited first and Hanzo to unlock Mifune ..