It has become very popular among LEGO builders to create pop culture characters inspired by LEGO BrickHeadz. This particular pair by toothdominoes is from an anime called Gurren Lagann, which features humans living underground, teaming up with mechs to battle oppressors living on the surface. Simon and his mech Lagann stand ready to face the forces of evil.
And such a perfect fit!
Fun fact: The eyes on the mech were custom made by the creator.