On the Internet, nobody knows you're a dog. As much as I like dogs, I am not one myself. But the line does best explain the point I'm trying to make: you can be whomever you want to be on the Internet.
Is there a problem with pretending to be someone else on the Internet? For instance:
- Lying, or at least Geneivas Da'as - To a certain extent this would seem permissible as per the "preventing harm" permission, but how far can you stretch that?
- Say a Jew pretends to be a non-Jew and is invited to a chat on Shabbos. Now what? If he wants to keep his identity intact he has to give some sort of a lie. Perhaps one shouldn't have a fake identity in the first place, lest it lead to a situation in which he ends up being stuck between pursuing his previous "white" lies with more serious transgressions or telling a bigger lie. Although the choice is obvious, to a person who's so caught up in this persona of his, it may not be so obvious, and he might c"v pick the former option.
- Beged ish/ishah - This is only somewhat facetious. If the problem is intermingling with the opposite gender (Nazir 59a), then would the same logic apply online?
- There are actually cases of people who in real life become their online personas, which could lead to them violating many mitzvos depending on the case. It's a real mental condition, though far from the halachic definition of a shoteh (they're still in full conscious control of their faculties). Like the sub-bullet point under "lying" above, perhaps one shouldn't be allowed to develop an online persona if there's a reasonable concern that he may become it. (This would, of course, not apply across the board.)
- Or worse - there are also cases of people who try to keep this up long enough that they develop multiple personality disorder, which could very well devolve into a halachic shoteh.
- Anything else I didn't think of?
Now for the meta/clarification/avoiding-being-closed-as-dupe information. Basically, unless you're a mod or a close-vote-hunter, you can skip the rest of this post.
This is not the case of one who creates accounts to boost his business. This is a person who has one account online who actively uses it to pretend to be someone else.
This is not the case where one pretends to be my elderly granny neighbor down the street. I am discussing a case where one makes up a new identity just for the fun of it, perhaps as an escape from reality.
This is also not the case of the 5-year-old indigenous New Zealander who wants to get more upvotes and answers by pretending to be a cute little kiwi. He's not trying to manipulate anyone. He's just wants to go by another persona just for the fun of it.