You'll be close emotionally -- maybe too close. You could become immersed in your friend's life, which could be a tricky proposition unless they're in the best of emotional health. You'll always try to figure out what they're thinking and feeling, what they need to feel better and so on, but that will be difficult because trying to see through them will be like peering through smoke.
Try not to get so caught up in this friendship that you start subverting your own needs in favor of theirs. While you'll be emotionally invested in this pal from the start, you might not know how they truly feel about the friendship -- and they might not either. You'll feel you really know this person, but it's very possible that you'll never really know each other at all.