Mei was my second rare, and I've spent quite a few scrolls on Kage with absolutely no sign of Darui.

I did hear from somewhere one theory: that on certain days you're more likely to pull certain characters. As in, every day each treasure has a hidden value that dictates which character is most likely to be pulled that day. So, assuming that's true, if you see someone else pull Mei then you should try pulling and hope for the best. I have never seen this actually happen before though, so I can't confirm this.