Make dashi: Combine kombu and 6 cups cold water in a medium saucepan and bring to just under a boil; remove from heat. Using tongs, remove kombu; discard. Sprinkle bonito flakes into the pan and let steep until they sink to the bottom, about 3 minutes. Strain liquid through a fine sieve. Dashi can be kept in an airtight container in the refrigerator up to 4 days.
Make dipping sauce: mix together dashi, soy sauce, mirin, lemon juice, ginger, and daikon in a bowl to combine.