Unfortunately, Japanese BBS's are not as reliable as I would have liked. The one I was looking at didn't say that Roboko and Muffler were just nicknames, and I didn't exactly look at the character pages of their source (because I got sidetracked by the other information on the website)...
Muffler is the nickname from
this site given to the character standing next to Aegis on the official website's character page. Apparently, the BBS I was looking at was expecting Muffler to be revealed, but he was not.
Also,
this is somewhat interesting. Its talking about the preorder bonus, which will be a digital media cd. It will include, among other things, some wallpapers, a screensaver, the opening theme song (Burn My Dread. OH ENGRISH), and voice clips... the interesting part is that the last voice actor is listed as "Secret Character" ... what this means is unclear. It could just be that this voice actor is for a character that has yet to be revealed, or it may actually be a secret, optional character. That voice actor was also responsible for the voice acting of the main character in Persona 1's Drama CD, for what its worth.