Re: Kabuto's 6th Casket, The Elder Brother, and Tobi's Identity
the shisui=tobi theory is good.
that could also mean, itachi didn't really kill shisui but he saw him commit suicide or something that would trigger the awakening of his MS. the suicide letter could be authentic and it's shisui's way of defecting from konoha.
shisui could be closely related to madara. he could be a son or a grandson, if he really is tobi. he knows a lot about madara. and he is nut crazy about madaras power.
this is just an indulgence of overthinking the theory...