Whimsun is a cheap card with the ability to gain +1 Atk for every ally monster on the field (card such as bomb and dog residue should contribute to this) giving it a maximum attack of 4. The changing number of cards on the field does not effect the stats of this card, as the abilities of a Battlecry activate once; which is when they're placed. One shouldn't typically keep this for late game however, since it has low hp compared to more expensive cards.