The sweet rice wine that makes up about one part of tentsuyu sauce is mirin.
A common all-purpose tentsuyu ratio is three parts dashi, one part mirin and one part soy sauce. Mirin contributes sweetness, gloss and a rounded savory flavor, balancing the salty soy sauce and umami-rich dashi served with tempura.
Mirin is a Japanese cooking wine made using rice, rice kōji and alcohol. Traditional hon mirin is produced by allowing enzymes from kōji to convert rice starches into sugars while the mixture matures. It is sweeter and generally lower in alcohol than sake, which is why it is especially useful in sauces and glazes.
Tentsuyu is not a single immutable recipe. Proportions vary by region, season and the food being dipped; some versions substitute sake or adjust the soy sauce. Mirin is also different from shio mirin and mirin-like seasonings, which may contain added salt or very little alcohol.