There is a very real and deep sense of support and understanding that will exist between you almost from the start. You'll instinctively respond to your friend in a positive way, and they'll feel as if they're completely accepted and well-liked by you. There will be an open flow of communication that runs between you -- and it won't necessarily always be verbal.
You'll simply get along well and will both enjoy your companionable silences as much as the stimulating conversations you'll share. You'll also encourage this person to be more social; if you meet face to face, you'll surround yourselves with mutual friends and loved ones. Essentially, you could be one of your friend's biggest fans. You'll also be able to support them through the rougher patches that will inevitably crop up in life.