ok, well a new character named Yuuki is sort of introduced the last 5 episodes, and she's like super duper good and all (aka the best player in alo). She's what they call a full-dive player (she never logs out) and she asks Asuna to help her along with her guild beat this boss or whatever before their guild disbands forever. Of course Asuna helps, and her and yuuki grow close. But yuuki doesn't want that, so she actually logs out. Come to find the reason that yuuki didn't want to get too close to asuna was that she was sick with an illness and her days were numbered- as were the other members' of the guild. The reason she stayed logged in all the time was because she felt stronger in ALO and more alive, because in the real world she is hooked up to a machine 24/7. This machine was designed to cancel out any pain a patient has and to allow them to feel and see real-world sensations, without leaving the room. Asuna, being the person she is, finds a way to help Yuuki fulfill her dreams through this machine before she dies. Yuuki eventually dies (and they make it super sad) and asuna solves her mom problems that she had using Yuuki's advice and it kinda ends there. They make you super duper attached to yuuki so its super depressing, plus the fact that shes been a sick kid like that her whole life. 

And that, my friend, concludes my longass explanation of the end of SAO2 XD THE END~