Fullback is a position that is mostly extinct. Most teams don’t even carry a fullback. They mostly just use a tight end if they need a lead blocking back. Roosevelt Nix is a true fullback though and is versatile enough to be useful to the Colts.
Nix played the first five years of his career with the Steelers. While in Pittsburgh, he made the Pro Bowl in 2017. He only has four career rushes and one touchdown to go along with those. He also only has 12 career catches that total 69 yards and one touchdown.