No, Kumirei is not a part of the staff, just a local celebrity. I think they actually started WK max two years ago.
There are a couple of personalities on here that have drawn quite a bit of attention to themselves through a lot of activity on the forums and various other things - in Kumirei’s case mainly the scripts.
If you want to find out more about the WaniKani lore visit this thread.
Edit: Kumirei’s profile says Joining date end of 2016, but I’m not sure if that’s when they really started.