Backtagging/Threadjacking : Go for it
Romance? : It's complicated. Someone already has feelings for her canonically but she is far more interested in finding her head than anything. So you can flirt and whatnot, but don't expect her to appreciate it.
Bad touch?: You want her to chop YOUR head off?
Hugs?: Sure uh... but she'll be awkward and probably won't return it.
Combat?: Sure thing, but she's a vicious little thing, just fyi.
Kidnappable? : Veeeery unlikely.
Death? : She... I ... don't think she can even be killed...
Mind Control/Abuse/Torture?: Eh, she sort of lacks the emotions that would make this particularly effective... physically on the other hand, if you have some means of restraining her? I'd rather discuss it beforehand anyway.
Fourth Walling/Meta/Powergaming?: No on all counts
#1 - Unless you A) Know Celty is headless because someone has told you or because you've seen her take her helmet off or seen it get knocked off, you won't know she's headless. You'd just think she's a strange person in leathers who never removes her helmet in person.
#2 - She's an Unseelie Fairy without her head - she is extremely private about her nature and her past - partially because she doesn't know it.
#3 - Because she has no head, she communicates via a PDA when in person. When using the SFC she'll probably use her handle "Setton", and only Text posts, so people won't know that Celty = Setton.
#4 - There is no #4
#5 - Celty may possibly ping as a darkness type of creature - since her element is shadow, and some people do consider her a monster...
#6 - Medical Notes: Celty has no heart. She has organs, but they do not function. She has blood, but it doesn't travel through her veins - she can live through her own autopsy, though it's hellaciously painful. She has a higher tolerance for pain than a normal humans. She heals from any injury, and most importantly - he has no head. :D