I like the nuance present in Yuko's character. Obviously, she's the devil, with the deepest abyss by far, of anyone in the series. However, that abyss only came about in the first place due to her family circumstances and literal child prostitution. In a way, I don't blame her for what she became. Even when she "manipulated" Esemori to come save her by kicking Uryu off, I don't think that was all ulterior motives from her; I think part of it was that she still deeply loved him, although in her own broken way, given what she's had to endure at the young age of 16.
She's without a doubt the most well-written character in this series and definitely this manga's Magnum Opus, imo.