To me, "You are the only person I know who might help me" sounds as if, in the author's circle of friends, the addressee is the only person able to help. Saying "You are the only person who I know might help me", the author is sure of the addressee's ability to help.

These two setences convey different meaning. The first one sounds like you're on a train full on strangers and you see this nice looking person and you think he's the only one on this train willing and able to help. The second one is more like you know a bunch of people and there's only one person out of all the others who is willing and able to help. I would assume you mean the second scenario. So yeah..

Hi Jordana, As the others have said, the meanings of these two sentences are slightly different. The words after "who" tell you what the action of the sentence is. and the words before the "who" tell you what you mean by "who". So in your first sentence (You are the only person who I know might help me), you are talking about knowing that this one person might help you. You may not even know this person. They might be a stranger, but for some reason you know they might help you. In your second sentence (You are the only person I know who might help me) you aren't saying you know they might help you, just that they might help you. And you are just talking about the people that you know, not strangers. And this one person, might help you. Native English speakers often make mistakes with this kind of thing.