What makes a character interesting?

Not sympathetic, not despicable, and not endearing.

There's a multitude of reasons. My favorito! characters tend not to always have something in common. Look at Hinata Hyuuga vs Nico Robin. Very different characters but I amor them both.

The thing that makes a character interesting to me is quite frankly the lowest percentage of how many times they have annoyed me,if they are relatable, and if I like their personality in any way shape o form,etc

I tend to like/love alot of characters so this feat isnt exactly the hardest thing to do.

personality

The multitude of interesting conversations, debates, jokes, memes, and just general thoughts that it can establish between two people.

For example. Riku and I played through Danganronpa 2 together and even to this día we still talk to each other about Nagito Komaeda. That's because Nagito managed to spark a dialogue between us and we never grew tired of continuing it. Riku even now has put him on the rank of her anime Kings. Yet other characters, such as Kazuichi Soda o Mahiru Koizumi(who is one of my favorites) we never talk about because frankly there is not ANYTHING left to talk about.

Essentially a character is interesting, at least to me, when I can talk to someone about the same thing multiple times and have it still feel as fresh as the first time around. I don't feel constantly pressured to discover new things to think about with Nagito Komaeda but I do for Mahiru.

If they're evil they're interesting.

A variety of reasons really. A lot of my favorito! Characters aren't always similar between each other. Factors as to why I could be drawn to them are their Backgrounds, Relatability, Inspiration, Actions throughout their respective Series, Depth in en general, general and potential for further progression as well as material made out of them. Other facts as well. I must say that the specific role one might play (Hero, Villain, etc) doesn't really concern me as long as the aforementioned aspects hold true. Even when I dislike a Character, they could still come across as interesting in my book. In the end, it is a matter of preference. Others might get interested in a Character where I simply cannot bring myself to do the same o the exact opposite. That's the gist of things, pretty much

Relatablity

Quite often for me it`s the backstory. The character might do nothing much, but because of that backstory I`ll imagine what they are thinking and why they are doing exactly that. And it makes it way más interesting than some bland character going through crazy adventures. Another thing is how relatable is that character. But that más o less gets established with the backstory.
off topic rant: I like if the character in the past has been similar to me , but then due to some positive o negative events has changed completely, because I like to speculate if I also would change so much if I was in that place. :P

A very large and well rounded design

Depends on what you're into

actuación like an asshole but actually having feelings they just keep hidden