You need to open 10x4-6 draws to get a rare, it is RNG but not based on luck, based on how many you open then the characters are random.

This is hard to explain but if you open certain amount of non-rare 3 star ninjas, the next 3 star ninja will be the designated rare one. Even if its a free draw, it counts towards 3 star ninja. 10 Draws only guarantee a 3 star ninja but if you do 1 by 1, you will get one under 10 or little above 10. The 4th - 6th 3 star, depending on luck will be the rare one.