When Anna appears at the flame inn you can combine your spirits with the vase placed in the Town History Museum. Since available spirits are different in hawk and wolf, try different combinations while exchanging spirits with your friends.
Spirit of Fire can only be gotten through the following method.
You must maximum have 19 spirits to get Spirit of Fire.
Push these buttons at the title screen.
Right, Right, L, Left+R, Down, R, Right, B. Once you hear a sound, just press start and visit your spirit list. Spirit of Fire will be only level 1.