Bonito stock gives the soup a deep, rich flavor.
The essence of Japanese soup, dashi is a flavorful broth made from simmering kombu (kelp, a kind of seaweed) in water. Wipe it with a damp cloth before using, and cut into strips with kitchen shears to extract maximum flavor. Shiso is a member of the mint family.
When it comes to creating a rich and balanced soup, the trick to keep in mind is always make sure you're developing layers of flavor. This starts with the broth and should be carried through to the garnishes.