How do I get better ending?

No not really. Just the text they would have is omitted or taken by another. The path you take gives you the end and whether or not Io(or others...if you can let them die) the path is the same.

I am not sure, but one of the achivement after you beat the game is : Befriend with everyone except the Anguish One and find a new path. So I supose to unlock this and you have to be maximum fate with every one so I think that everyone need to be alivre. But I don't know for sure, I only beat the game 2 times since now.

Well, it depends of what is a "Good Ending" in this game. There are 4 plot related endings to this game:

Yamato's route: Join Yamato in his pursue of The Merit System
Ronaldo's route: Join his ideal of the World of Equality
Daichi's route: Join him, defeat all other parties, and finally defeat Polaris to gain True Freedom.
Anguished one's route: Join him to defeat Polaris, so he can take control and restore the world as it was.

All other members are unrelevant to the each ending (I don't see how you can miss to save any of them)
That's independent to the Fate level of each character related to a route.

there a way for better ending which is you could get from Daichi's route.
you need to get all people fate meter to 4 or 5 except Anguished One and you need to chose to return the world to the past and not to destroy polaris