You'll never get officially sanctioned ones. The VN3 specifically says R18 is prohibited and I believe Ruri is about ~15 years of age canonically if what I read somewhere is correct. There have been some outfits that were riding this line that were pulled about a month after their respective releases last year around this time, including a couple of swimsuits that were just a -bit- too revealing. Would not recommend doing yourself either... it feels wrong and it's just wrong altogether.
If you really want to emulate Ruri, there's plenty of 2D-anime-looking avatars that exist with mochifitter forward-only profiles and R18 allowed in their respective VN3 licenses. You can just convert from Ruri to those as Ruri's first-party profile is forward and reverse.
