The Beastector is the group composed by Lunar, Tarus and Merco. They were humans that were transformed into beasts and gained amazing fighting abilities. They work for Evil Emperor and are his better gangsmen. It's unknown if they remember they were human, but it seems they do not until in the end of the game when Evil Emperor is defeated.
It is clearly that each member is based on one animal and thus has their fighting abilities based on it. Lunar, the wolf one, has very fast moves. Tarus, the gorilla one, is strong enough to throw boulders at Marina. Merco, the falcon one, has amazing aerial moves. Out of the three, Lunar seems to be the leader.
Plus, each member got one vehicle. Their vehicles have names that are based on mystical creatures. Lunar's motorcycle is named Cerberus α, Tarus' tank is named Sasquatch β and Merco's flying drill is named Phoenix γ. Each vehicle can transform into a moving robot which appearance looks like the same animal of its owner. The three vehicles can unite to form up a giant robot.