User blog comment:Gsimenas/Volume 8 Yen Press naming/@comment-28251580-20160828101837/@comment-6697950-20160829172434

Kawahara hasn't given any official English name for the character and both versions are technically possible transcriptions. As long as you understand that the "r" isn't silent.

For interest's sake, I did a search for the Japanese name and it led me to https://en.wikipedia.org/wiki/Koper. Other entries include "copel" and "coper". No entries for "kopel". So, "Coper" is more likely to be the intended spelling than "Kopel".

So, I vote for keeping "Coper" for now. Maybe ask Kawahara if he has a spelling in mind.