I prefer
Rainier, but
Rainer's good too.
Karl Rainer - reminds me of something; don't know what. Nice enough.
Rainer Koloman - oh hell yes. This is just glorious in its Germanic-ness.
William Rainer - looks a little too much like a FN/LN to work entirely, imo
Rainer Lawrence - I prefer
Laurence, but this is very, very good.
It seems versatile enough to me, and certainly works with non-Germanic names. I like it :)