It's all about luck. Of the rares (Mei, Danzo, Kazekage Gaara, Darui, Onoki, Chojuro, Mifune, Killer Bee Seven Sword) there's an equal chance of pulling any of them; none of those listed have a higher chance than another.

As for myself, it took about 840 scrolls or so, and just before getting a super rare (along with 6 dupes of others in total I think), to finally pull Mei from the Kage Treasure.

Best of luck!

Edit: I should note that I have group mates who pulled her as their first rare; I was just incredibly unlucky.