You know I have a theory....but I'm gonna tease you a bit first.
The writers have revealed that this season they will introduce Henry's father. They have also promised a reunion of Baelfire with his father Rumplestiltskin (Mr. Gold). It's completely natural to jump to the conclusion that this man is one of those two. .... or both. While I still fantasize about Bae being Henry's father, much to the distaste of some in the ONCE fandom, if you depend on the ONLY true predictability of the writer's true nature, you know that they like to be unpredictable. Which is why I insist it is not Bae.
Now I'm going to blow your mind. The opening mysterious man is The White Rabbit.
In the sneak peek for Season 2 Episode 2 "We are Both", we see Jefferson talking to Prince Charming (David) that is a portal jumper, and his hat was his only portal. Which leaves me to question: Are there other portal jumpers? Of course there has to be!
In the end of the episode Hat Trick, His fairy tale history finds him trapped in Wonderland, and by the end of season one, we still have no idea how (or if he even did for that matter) return to fairytale land. We know, from the classic story, the Jefferson was present in Wonderland when Alice came through. Her appearance, outed to Jefferson that there is a portal jumper in wonderland when Alice reveals she followed The White Rabbit.
It is just my theory that this mysterious man, the white rabbit, is, infact a portal jumper (and I have the inclination his portal may be a pocket watch). He helps Jefferson escape to back to fairytale land before the curse takes place, which is how he ended up in strybrooke cursed. But it's also my theory that Jefferson and the White Rabbit had many meetings between his return to fairytale land and the curse taking effect- and hence The White Rabbit and Jefferson co-hearse a plan to help return to fairytale land after the savior breaks the curse (Because when the curse took place, Regina still had Jefferson's only portal--the hat).
I know what you're thinking...... "you're crazy! There is no proof of that!"...But anyone following the show knows that the writers LOVE to drop you all kinds of hints.....Are you ready for them?
Clue one: Opening Song. It's very unusual for the show to deviate from show's score. For some reason this stranger is accompanied by the song "Charlie's girl" by Lou Reed. Perhaps to subtle to make the connection; but Alice's Adventures in Wonderland is Written by Charles Lutwidge Dodgson, aka, Lewis Carroll.
Clue two: He travels underground (the subway). Now I know it's a stretch, but I also know that going into a hole is not a first for a rabbit, and we know the most famous hole the white rabbit went through in Carroll's book. Why not travel by holes in the ground?
Clue three: The sign in the apartment."Cleaner and Hatters".... I'm not even going to go into immense detail here, but I speculate that the sign has some association to his relationship with Jefferson.
Clue four: The Red Hand and the Heart pictures. At first sight, some speculate that the red picture (above) depicted red riding hood, but upon further inspection, I believe it is a red hand with fingers enlaced with scrollwork. It's not detailed enough to depict what the scroll says, but I just have the feeling that it has to do the the queen of hearts with leads me to the second picture near his window; a drawing/sketch of a human heart (below, near the fan by his shoulder).
Clue five: the apartment contents. clocks (duh), cameras record player, and everything in that apartment seems so outdated, it really doesn't make sense that a man is in a trendy updated suit, with an ipod (or iphone) would live here. Some fans have stated it looks 1970ish, but I think it's further than that: 1950s. Rotary phones, non-ditigal clocks, and leather cased cameras all come from the 1950s...and why is that crucial? Well.....this is ABC afterall...Disney property and all that........ And Disney's animated film Alice in Wonderland, came out in 1951.
In my theory, The white rabbit, who has not visited our world for a very long period of time since the 1950s, came to our world about the time Emma is to break the curse to help everyone return to fairytale land. Once the curse was broken, Jefferson, sent a postcard to Mr. Rabbit (see what I did there...) so he would know where they are.
Now I know that, for certain, I could be dead wrong. I could be seeing what I am believing, and not believing what I'm actually seeing (but if we find out I'm right about Dr.Whale being Frankenstien, you are going to have to deflate my head a bit). Regardless, Once will take turns and twist that will surprize us along the way, which is why we watch it in the first place. Whether he turns out to be the Rabbit, or Bae, or Henry's father, the theories are not important; They are just part of the fun!