Songbird

Him is a 30-foot winged creature that is feared by the citizens of Columbia, and is the guardian of Elizabeth

History
When Elizabeth was imprisoned in Columbia for 15 years, a creature, simply called Him, was created for the sole purpose of keeping her in prison. During that time, Him was Elizabeth's only company. Although Elizabeth was, Him had been her caretaker and protecter. Eventually, Elizabeth grew to love Him as her only friend. When she had escaped, Him was intent to bring her back, even if it meant destroying anyone and anything in the way of Elizabeth, including Booker DeWitt. Little is known about who created him, but one thing is certain: the armored suit of Him resembles that of Rapture's Big Daddy.