Some of my favorite characters that I remember reading were Harry Potter, Hester Prynne, It, Cujo, some others that my cold induced head can't think of right now. But as you noticed some of my list above were antagonists.
Are they okay to be Memorable characters? I think so because they are memorable for being so bad. Some of them aren't even human, which is also okay since they don't have to be. What about the rats in Secret of Nymph? I think there are some in there but I can't recall any names at the moment.
I think one thing that makes a character memorable is the fact that they have something you can relate to. Either a fear, or a skill that you admire.
Do you think Cujo wouldn't have been a success if the fear of dogs isn't a very common human fear? Do you think It would have been scary if people didn't fear clowns? I'm not afraid of either of these things, yet these antagonists were still scary. Cujo because he was uncontrollable and smarter than some dogs. Not sure if it was the rabies, or something more. It's been a while.
Harry Potter is memorable because he is the awkward kid that sticks out, we've all been there. Hester Prynne might be harder to define, but it still boils down to her being an outcast. I can get behind that sort of character. I like to root for them when the odds are stacked against them.
What about character makes you remember them?