He shows emotions in a wide variety despite being a demon and is deeply intelligent and able to do things will with an air of confidence that shows little fear. But he's got a lot of other traits that makes him so likeable and enjoyable to watch. Well I think everyone in the history of the black Butler fandom can agree Sebastian is good looking. I honestly just love how he acts when he see's any cat because it's just adorable (Am I wrong?) and I love how he handles a lot of the situations he is in with a smile and without a single complaint. Not just because of his looks but also because of how he handles things and how his emotions are portrayed.
Sebastian is, of course, a fan favorite to many.