I think that she is pretty in the manwha because
1. the characters drawn by the artist are always pretty
2. the artist didn't know she was described as plain (I kind of doubt that)
3. the artist couldn't bring him or herself to make her ugly
or 4. her eyes were the prettiest part of her right? so the artist drew her with really nice eyes and since most manwha, manga, etc. drawings are pretty or ugly depending on the eyes, she appears to be really pretty in the manwha but plain in "real life" because in real life, the whole face matters- it's kinda hard to draw an ugly face with pretty eyes.