Great thread idea, HI!
The difference between my real life identity (H****) and my online personality is complicated by the fact that Estelyn has two different aspects to her: she is the person who posts and moderates on the forum today, but she is also a character who lives in Fourth Age Middle-earth and takes part in (RPG) adventures there.
Similarities: Both H* and Estelyn are female, both no longer in their youth. (Though the online person is quite reticent about giving her exact age, since numbers can be divisive, and it's nice to cross age boundaries on the internet.) I think that the way Estelyn reacts is fairly similar to H*'s reactions. The moderator Estelyn does weigh her words more carefully than the poster Esty does, though. (Hmmm, even the change in nick, cutting it short to Esty, seems to change things - I don't use the abbreviation in my official function.) Both Estelyn and H* are reticent about showing too much of their private side to others unless they feel safe.
It's easier to define the differences than the similarities, I think. I would agree with Saucepan that the online person has the advantage of proofreading conversations, so that mistakes made by spontaneous utterances are few and far between, hopefully. There's also a major difference in language - Estelyn writes in English, H* speaks German almost all of the time - that certainly influences the choice of words and sentence structure.
Estelyn writes a
lot - H* very little. The latter tends to forget to answer e-mails and writes so few letters that she is surprised to find out that the price of postage has changed again. Her journal is written very erratically, and she never writes fiction. Estelyn is involved in RPGs and spends hours crafting posts for her characters; furthermore, she has even begun online journaling, with the result that there are more recent entries there than in her private, hand-written one.
The RPG Estelyn lives a much more adventurous life; though she has her home base in Minas Tirith as the Head Loremistress of the Library there, she travels a lot to gather stories. Actually, now that I think about it, that's a similarity - her real life creator likes to travel to gather impressions and ideas as well!
And then there's the chat Esty - she's a combination of H* and Estelyn - a bit more daring and less inhibited than in real life though, with a tendency to enjoy mild flirtation occasionally...
So, to answer the question HI poses - is Estelyn a mask that H* wears? I would say not - she's just part of a personality that is much more complicated than can be shown online. (The older I get, the less inclined I am to put on a show for the benefit of the public.) But maybe there's a bit of a mask, for who can show a naked face to all of the world?!