The day before it was caught, Marshal had Costa with him as a Tirtouga when Black and White saw him and Alder battling each other in the Battle Subway. After losing to the Champion, Tirtouga was released in Nimbasa City due to its disobedience.
Sometime later in Nimbasa City, it comes upon Black again, which it was battled and defeated by him and later caught and named Costa.
During the Gym battle with Driftveil City Gym Leader Clay, he fought against his Krokorok. After a few strong attacks, Costa ended up defeating Krokorok altogether.
Later during the Gym battle with Brycen, he evolved into Carracosta and won with AncientPower.
According to Marshal, Costa is very stubborn, being unable to obey any of its trainer's commands. Even with Black, it still retains the same characteristics.