User blog comment:Alena Tachibana/Bonds??/@comment-4866818-20120614234934

It wasn't made clear why Ruka saved Zero. However, in my opinion, I believe it's because she didn't want Kaname to sink to a low-life beast. Let's not forget how she was just talking to Yuki moments earlier, with Yuki saying she wants the old Kaname back to her, so this may have been Ruka's motive.