Well, to get Onion Knight, you need a friend codes and send messages to unlock it. However, I dont remember exactly how many messages you need to send

To find the cave with the Iron Giant, you must receive letters from the 4 old men and prince Allus. After you receive the letters, proceed to the secret dungeon using the Nautilus. From the southern tip of Dalg continent(where it looks like an arrow), submerge and head west. The location is halfway between the floating continent and the dalg continent. It appears as a shadow on the surface of the water. While underwater, look for a multicolored patch of seaweed.