You must be new to this game because they aren't going to swap your ninjas for you just because you want something different. Otherwise, we'd all have the ninjas we want. It's random, so you have to deal with the hand you're dealt. I wanted Mei and only got her after 180 scrolls while others got her their first 40-60.

My first 60 scrolls got me Darui, the next was Killer Bee, then Chojuro, and finally Mei. I knew I was close to another rare based on the number of free pulls I had done and took a shot. Luckily I got her because I planned on saving all of my scrolls. First time playing this game that an impulse 10 scroll worked in my favor lol.