Constable Odo is a shape shifter. His species, the Changelings, comes from the Gamma Quadrant on the other side of the Wormhole located near Deepspace Nine.
His people send children out into the universe to explore and report back. While Odo's species leads the Dominion - a totalitarian force looking to control the universe - he has chosen to remain allied with his Federation friends.
He and Kira became very close, though their budding relationship was cut short when he returned to the Gamma quadrant to help rebuild his people.
Like it? Buy it on Society6