Yoko is 100% untarnished awesome from the beginning in the two translated novels (Tokyopop better publish 3+ or people are going to die). I've only seen the first few episodes of the anime, but I think it makes a huge difference that they aren't in Yoko's close POV like the books, so all we get of her is how she initially acts, which is like the Good Girl Who Needs Everyone To Approve Of Her. If the anime is at all like the books, she does get mightily better, and a lot more cynical as being a Good Girl gets smashed back in her face. Which gives you the fun story of the universe trying to mold a Super Speshul Selfless Sue Heroine out of someone going NO, REALLY, FUCK EVERYONE ELSE EVER, BYE.