And now I feel really lazy and dumb. Took me all of 10 seconds to find a couple of these on the underside of leaves.
The cross-striped cabbageworm (Evergestis rimosalis) is a medium-sized caterpillar with four pairs of abdominal prolegs. They have distinct black and white stripes across their back and a yellow stripe that travels down either side of their body (figure 14). Adult moths lay eggs in masses with 3 to 25 eggs in each mass.6 When the eggs hatch, the host plant can be quickly skeletonized (figure 15). Caterpillars show a preference for feeding on tender leaves, especially at the growing point, but may also bore into developing cabbage heads. They grow to about 0.75 inches before dropping to the soil to pupate just below the surface.
Skeletonized describes the Tatsoi/Chijisimai.
Bit too late for companions but thyme, dill, oregano, lavender, onions, garlic, and marigolds can deter them. Spraying with Bt is an organic spray option. Nasturtium can be a trap crop. Repeated hand picking is what I'll do with just a few plants. I imagine they'll keep working their way down the bed to the bok choy. Everything out there aside from a few lettuce plants are brassica.