From what I have read women were very oppressed in Japan during the Edo Period. Were there any instances of women becoming samurai (including cases of them pretending to be male) or groups of women banding together to work as thieves/assassins/mercenaries/etc during that Period? Or even any women regarded as being "warriors" or fierce fighters?
Women were probably not as oppressed as you believe during the period. To quote an answer I gave several months ago to someone asking about the treatment of women in Edo Japan:
" more recent studies have painted a picture that's more typical of much of the world at the time, in that women in higher society were bound by more rigid rules than the common people.
In Japan... the role of women, legally speaking, was fairly similar, with the family being of a higher order than any individual male or female. Divorce, for example, was initiated not through the male spouse, but through the coordinated activity of the families on both sides, and both families had equal standing in the marriage, with both spouses having the ability to remarry. An especially intruiging case involved a woman called Sui from Jōshū, who requested rienjō (documents related to divorce) three times on her own initiative."
I'm not sure of any recorded instances of women thieves or mercenaries, but women did have their own warrior "class" (as part of the bushi class) in the onna-bugeisha. For the most part, female involvement in conflict was of a defensive nature, but most women of the Samurai class were trained in some form of martial arts for the purpose of defending their home or castle. Tomoe Gozen, Nakano Takeko, andHōjō Masako are famous examples of onna-bugeisha.
From this point I will direct you to this past thread regarding the onna-bugeisha, as I'm afraid my expertise more in the civilian side of things.
https://www.reddit.com/r/AskHistorians/comments/20n7uo/how_common_were_onnabugeisha_female_warriors/