I consider it the same as art theft. You're taking someone else's creative property and using it without even bothering to ask permission. That could be someone else's original character that they designed and created, or someone else's original character that they commissioned or paid for. Even if you aren't claiming the art as your own, you're claiming the design as your own- Not as in, "I designed this." but "This is MY character's design." That's hella rude. You shouldn't be using anyone's creative property without their expressed permission.