Same here... I got two theories but since I really really don't get enough of this at all, all my attempts at theories will probably end in some logic felony...:/
But if it's not a completely new character, I think this is either the Handmaid or a really weird antithesis of the Sufferer.
What made me think of the handmaid is the chumhandle, which just reminds me a bit of a doctor, her use of U_U smilies similar to Aradia's smilies with 0_0. Also this character writes in a british way, maybe some kind of influence from Lord English (if he's indeed English)? But this could also be explained with an obsession for the letter U.
However, The character revealing she's a prospit dreamer, speaks against this.
What make me think of Sufferer is the colour of the font ( a grey slightly brighter than Karkat's) and that these conversations mimic those of Karkat, just with an EXTREMELY FRIENDLY

tone all over it. The Sufferer was known to be friendly to almost unbearable levels until he completely flipped out at the end. The presumely female gender of UU doesn't go well with this theory though.
So maybe it is just a new character?