Kroma

When it appears

Once you've completed the main quest Destroy Vados and taken Eliza up on her offer in Hilda's Room, and after you win Hilda 2 and the story brings you back to Outpost, it can turn up in the wild.

Capturing

It has no favourite foods, so each food adds 1 to your capture chance, and Perfect Berry 10. As a Legendary Nexomon, it's harder to capture: food counts for less, Nexotraps count for half, and wearing it down adds at most 10 instead of 20. A Golden Nexotrap still adds the full 100. Catching a Nexomon explains how the whole chance adds up.