Charizard is probably the most useful support Pokemon that you can unlock in the game as he can provide you with many different support items such as Training Soda, JP Power Up, LP Restore, Skill Restore or lots of coins. He will cost you a whopping 600 Diamonds to unlock, but over time he should pay for himself with the amount of goodies he will give to you.
You can find Charizard by going to the Town and entering the Diamond Shop and selecting Friendship Items. The Item you need to aquire is Charcoal, that will unlock Charizard and he will appear in your pond with the abiliity to grant you a free item once every 12 hours.