Noblesse: The characters are simple, but developed enough. As for likeability, the F' and Rai duo are great. Rai's character as an unrelenting righteous super being isn't all to new, but it is presented well, including an intriguing back story. But F's character in the beginning is a humble servant, and the more we know about him, the more he trolls and becomes a fun character. As for the others, well, the teenagers pretty much don't even matter anymore (they were all 2 dimensional anyways). The trio hasn't changed or grown as characters much, beside M21. The other nobles, pretty much remain 2 dimensional. And the villains are mostly one/two-dimensional characters, not even worthy of being remembered.
Lets just do main characters. Rai is driven by his creed, of doing no harm but to those who do harm to humans. At least, as far as we know, he has always been like that. He likes to keep up pretenses and remains relatively stoic all the time, almost as if he has little emotions besides happiness and loneliness. That makes you wonder if he is just like that, or if something caused it.
Franky- A former mad scientist, now turned principal. He is seen as compassionate, but is actually a huge troll and battle crazy. But he's also a genius and excellent fighter/strategist. What else can be said? He is pretty much a genius that got crazy powers, literally.
ToG: Almost all of the characters have secret hidden pasts, that created who they were. And that pretty much drives what they become or do in the story. Most of the characters develop complex emotions, and philosophies. Most characters are like-able and identifiable.
Baam, has a somewhat tortured and lonely past, which becomes the driving force of his actions. Like Rai, he has a lonely past, until he met friends, whom he wanted to protect, he never knew loneliness. He bares a great power, but also a great burden. Its not that he is unwilling to kill people, but that he doesn't want to.
Koon: Just like F, he is an incredible genius and brilliant schemer and strategist. He is also powerful, but rarely uses his power, since he likes ordering people around. In fact, except for F being as powerful as the other nobles, Koon and F's characters are almost alike. Scheming, brilliant, powerful (relatively), characters who love to troll.