Just to add in, as a f2p player, I've gotten a good amount of ninjas from luck-based events.

May-100 puppets Sasori, in a single mission box (I wanted more Chikamatsu frags, okay?)

June-Tendo Pain, in a set of 55, 40 of which I got for free and I saved up throughout the week.

August-Tobirama Senju, in a set of 10 which I decided to buy (as in, use coupons) because why not.

September-Lucky Stars wheel got me my Kage Minato.