The cheracter of Vera in the story 'The open window' by Saki.
I think that it is completely clear that Vera is the mastermind who is driving this story. No one else really plays any sort of an important role in causing the events in this story to happen.
Framton Nuttel drives the situation by coming to Vera's home, but beyond that, he is not in charge. Only Vera is in charge. She is the one who is going to decide what stories she will tell. The stories she tells (first to Nuttel and then to her aunt) make the story what it is. No one tells her to make up these stories, so no one else is controlling what happens in this story.
So Vera is, without doubt, the mastermind in the story.
sorry for the spelling error...