The Cube.
The Nymerian Cube is a cube containing a small, collapsed star. According to A.V.A., it is extremely powerful. Hence why Clarence wanted to get his hands on it.
The cube is a driving force behind the events of "The Toro Regatta", in which Clarence hopes to steal the cube from racer Rug Yorkvain during the Toro Regatta. It had to be done during the race, since the cube is normally too well guarded to steal.
Gary Goodspeed and Ash Graven managed to steal the cube after Rug's ship was destroyed and Gary defeated Rug in a fight.
After the race, the cube was revealed to allow the Team Squad to communicate with the titan Bolo. He used it to appear before them, and instruct them to find the Dimensional Keys. It wasn't until later that Gary learned the Nymerian Cube was itself one of these keys, making this the only Dimensional Key that is given a name.
In "The Set Up", Sheryl Goodspeed used the cube to contact the titan Oreskis, for whom she was collecting the Dimensional Keys.
The cube was destroyed along with the other four keys when Mooncake and Nightfall freed Bolo.