The Japanese leaf named as a common tempura fritter in soba or udon soup is shiso.
Shiso is an aromatic Japanese herb in the mint family, also known botanically as perilla. Its green leaves, called aojiso or ōba, are often battered on one side and fried as delicate tempura. The crisp coating contrasts with the leaf’s fresh, minty and slightly grassy flavor.
In noodle dishes, shiso is one of several possible tempura toppings. Hot soba or udon may instead feature shrimp tempura, kakiage—a mixed-vegetable fritter—or other seasonal vegetables. That variety explains why shiso is a specific answer rather than the name of the whole noodle dish.
Shiso is also used raw as a garnish, wrapped around food, or paired with sashimi. It should not be confused with wakame, a seaweed commonly added directly to noodle broth, or nori, the dried seaweed often associated with sushi. Mitsuba is another Japanese herb, but it is not the leaf identified in this tempura description.