One of the aspects of writing characters like them is that if we’re not going to get viewpoints from them, their personality has to be strong enough to manifest externally. Which tends to have an effect, if it’s not done well–or sometimes even if it is done well–of making them feel one-sided. In some ways I play this up; for instance Nightblood really is one-sided because of the way his personality works, the way he was crafted. He’s a construct, and he has a main focus.
So with someone like Syl, I really wanted to bring out a lot of personality in her dialogue so that we could characterize her without having any of the internal thoughts and monologue and emotions that I sometimes instill in other characters. But Syl also was meant to be a vibrant splash of color in Kaladin’s sometimes dreary viewpoints. Because of that, I really needed her to just pop off the page. So it was done intentionally.