She was a Spartan who died trying to protect Halsey before she was kidnapped in what eventually led to the CG Legends short (I can't remember it's name). You learn of this from one of the Adjunct sections in the reissued Halo novels.
If I remember correctly, I think Sheila survived on the Pillar Of Autumn (In a coma-like state) and was found by the chief. I think she ended up on Onyx with Fred, Kelly, (Another Spartan II?) and some Spartan IIIs.
I haven't read the books in a while, but I'm pretty sure Sheila was alive. But hey, I could easily be wrong.
Oh, yeah, absolutely love the comic by the way! I hope I can develop art and stories like this eventually.