as someone that's been in this situation before, i pm'd them and politely let them know that while i'd like to reply, i didn't have any info to go on in terms of who it was or what they'd be like and what might be the best way to set something up. they had no profile info, no name in their name field, nothing on their journal, and i had no way to who/what i was tagging.

i was fortunate in that they got back to me nicely and directed me to a wiki about their char, and then posted something in their profile after. it's a bit of a tossup what someone might do. they might not get back to you or they might give you a basic run down or they might just decide that means you don't want to play with them and they remove their tag (which i've also had happen). just be polite but upfront and the rest is up to them.