Cobalion is the leader of the Sacred Swordsman and was one of the Pokémon that helped
Keldeo learn the true weight of its sword to become part of the Sacred Swordsman.
Cobalion pretty strict, but is also very wise and sometimes speaks it in riddles to help his pupils learn on their own.
While attempting to help save
Keldeo from Kyruem, it was frozen in ice. It wasn't until
Keldeo returned to face Kyruem again with
Ash Ketchum and his friends that it was released from the ice. After
Keldeo proved to be worthy to become a Sacred Swordsman,
Cobalion got
Ash Ketchum and his friends to be witnesses of the oath.