Answer: St. John
When St. John receives a letter from a solicitor named Mr. Briggs, he tells Jane that it is imperative that a Jane Eyre is found, as her Uncle John has left her an inheritance. He also tells her the story of an orphan girl who became the governess at Thornfield Hall, and then disappeared.
Jane guesses that St. John has already guessed her identity, so she reveals it to him. She then asks him how he knew. St. John shows her a scrap of paper with her signature on it. He also explains that, although he had not realized it before, he is her cousin, as her Uncle John was his Uncle John, and his name is St. John Eyre Rivers.