It seems the OP might have an Azure damsel...a blue one with a yellow belly. It's supposed to be one of the less aggressive damsels. I had one. It left everyone alone but it staked out a little territory and defended it. It's not as mean as a domino damsel which goes after every fish in the tank. It just protect its territory like clownfish protect anemones.
60 Gallon SPS dominated reef tank.