Early in the story, a Tenko starts living in your house. Speak to it and you can send it out to gather items for you. While it's away, start a mission on the same map (they're highlighted with a little Tenko icon in case you forgot which one you picked) and you should find it out on the field somewhere. Run up and touch it to unlock the trophy. It'll follow you around for the rest of the mission, helping out in combat.