I'm trying to come up with an OC, and she basically fights close-quarters with a wakizashi and a tanto. Was this a thing done in feudal Japan? Or was the daisho strictly katana and wakizashi?
For those who don't know, daisho is basically dual-wielding for samurai. As far as I could tell, daisho was strictly done with a katana and a wakizashi, which is basically a short sword. My question is, were there ever exceptions?
Daisho is, by definition, big sword\small sword. This is generally meant to be a daito and a shoto. Especially after standardization by the Tokugawa Bakufu. So, if you are wanting to be completely accurate, Edo period daisho could not be of a wakizashi and a tanto. Pre-Edo, the daisho was not the "badge" of the Samurai as we know it now so there were more variations...but the daisho wasn't really much of a thing, either. During the Sengoku Jidai, which swords were worn was chosen by practicality and not by regulations.
A DAIto would be a katana or tachi, which would have a nagasa (cutting edge) of more than 2 shaku. That is almost exactly 24 inches. That is the DAI. When warfare began to shift focus from cavalry and archers to more infantry combat, the tachi was phased out for the uchigatana. This later became the standard katana. Some uchigatana were shorter than 24 inches, but generally speaking, they were basically daito.
A SHOto is a wakizashi or tanto. Wakizashi, which came from the Sengoku periods "koshigatana" (hip swords), are anything with a cutting edge between a katana's 24" and a tanto's 12". There are wakizashi that are very large, almost katana, that are mounted like a katana. Sometimes the term Chiisagatana is applied to them, but the term is more complex than that. A chiisagatana might be able to be worn as the DAI in a daisho. A tanto is anything with a blade under 12", but larger tanto do exist. If it is slightly larger than the wakizashi threshold, but still of a shape and style of a tanto, it is called Sunnobi Tanto.
Now we run into a bit of an issue, historically speaking. The daisho bearing Samurai class was something that became the norm after the beginning of the Edo period. During the Edo period, women wouldn't wear daisho. Neo-Confucianism was in full swing, so the role of women in Japanese society diminished greatly. The female warriors, onna bugeisha, would have carried a small blade called a mamorigatana or kaiken. The naginata and yari were common weapons for onna bugeisha during the Edo period, and the katana (or even wakizashi, in this case) was generally only used by women during emergencies.
NOW THEN! After all that, your OC won't fit perfectly into the norms of Edo period samurai culture, when the daisho was a big deal...so I'd say just make the kind of character you want, not what history dictates you SHOULD have.