Transformation is the process of adding two of the same card together to make the card more powerful and change its appearance. Transforming costs Garu and is essential to the game. The only way to truly draw out the true power of a card is to transform it to its final state. To transform a card to its final state you need to have 3 or 4 of the same card.
For the three card method you put two of the same card together and then put the resulting card with a third card. This is not a suggested method, as the card will not reach its fullest potential this way.
The other method is to put two of the same card together twice and then combine the resulting cards. This requires 4 cards, but will help your girl reach her true potential.