I recall that SE enforces a three-letter minimum on usernames.
But, this doesn't seem to be the case here.
There's no Unicode magic going on, his username really is just one character long:
Looks like there's a bug somewhere :s
After some testing, it seems like there is no length check at all for unregistered accounts. And, when registering an unregistered account, the short username is carried over (Here's a test account I created and registered).