Yup. I had similar reasons for thinking it was her too. Though my main reason was that I got weapons only she could use before she died so surely the game wouldn't create a class of weapons just for a character that dies fairly early into the game.
Just some fun speculation about the 7th character from someone only about 10 hours into the game. Right now I only have 3 characters so obviously this is just going on a whim but...
Would the 7th character happen to be Fiora? I don't think it'd be Riki since people seem to be throwing that name around pretty freely on here. All I'm going on is the knowledge that the 7th character is surprising as a spoiler. I was thinking Fiora since she had supposedly died, and also the fact that she was made a playable character early on in the game. It doesn't really seem that surprising to me (if it is her) because it just doesn't seem like it'd do much good to make her a playable character if you were only going to be able to use her for a brief moment.
So yeah, am I completely off base here?
Even more spoiler-y below.
The funny thing is that she can't even use those weapons when you get her back because she has a new type of weapon set.