This marinade is Bill Kim's version of ponzu, a Japanese sauce made with soy sauce and citrus. Here he uses mirin, a sweet Japanese rice wine, and yuzu, a floral citrus. This marinade keeps for several days in the refrigerator, so make a large batch and use half on squid and the rest for tofu.
While the squid is tasty after 30 minutes of marinating, an extra half hour makes a big difference in the flavor. This supersimple marinade keeps for several days in the refrigerator, so it's great to make a large batch and save half for later.