I wonder if the author would do the same thing twice. A Supposed friend, who likes Yotsuba and wants to get rid of the main because of it, seems rather repetitive. Also if we think about a mastermind he wouldn't help someone who wants the same girl except blaming him at the end to get her.
Also, another time traveler would make the Story pretty Paradoxical, what would be the point of the Mastermind being a time traveler if that power would amount to nothing since he wouldn't change the path Yotsuba is on but would create a different one.
So I think the best bet is a Family member or some one envious of the main.