Qwenthur or Quenser?
If it's Quenser then why does this website have it wrong? And I'm certain VA's pronounce it differently. What gives? Is there any official answer?
Where did you get Quenser from?
Japanese doesn't have the "th" sound, so the closest approximation is an "s" sound (which is why you often year "sank you" in anime instead of "thank you")
Technically his name in Japanese is transliterated to KuwensÄ, but stylistically his name is spelled as Qwenthur, so that's how promotional material and the dub recognize it