The theory posited elsewhere that Leviathan will turn out to be Kathy (not Kate) Kane is, I think, worthy of consideration.
- Spymaster
- Worked with the Bat-family, but definitely an outsider
- Access to Spyral ("Spy"-ral... I just got that!) face-swirling tech
- Leviathan's outfit would sufficiently hide a female form, in a classic "Mask of the Phantasm" kind of way.
Also, I just plain like Kathy Kane