Personally, I'm doing Youtube for many reasons, but the main one is to be fully comfortable with myself. And I believe that people enjoy Youtubers who are genuinely themselves, and that's what differentiates TV from Youtube. I may had been trained in acting, but I will only play characters for a comedy skit or short film for my Youtube channel, never fully for my "youtube personality", that will always be 100% me.
I feel that an "on-camera personality" is basically someone who just feels comfortable being on camera and NOT pretending. Although there are some out there who are doing Youtube for the wrong reasons and those are the people who are mostly "fake", because people are really good at subconsciously knowing who's being fake.
Take Pewdiepie for example, on camera he's really loud, in person (from other fan videos) he's really quiet. Now people might think of him as fake or something but I think it's because Felix feels comfortable on camera, he's in his element, take him out and put him in a crowd, he feels out of his comfort zone.