Mesprit is a legendary pokemon found in the Sinnoh region. It appears in the cave on Lake Verity after the player encounters Dialga or Palkia. It does not allow you to battle it right away however. It flies to a random place in the region. To find Mesprit the player must use one of the apps on the Poketch. Mesprit is encountered at level 50 and is a Psychic type. It is one in the trio of the legendary spirits. Its other counter parts are Uxie and Azelf.
Pokedex Entry
Known as "The Being of Emotion". It taught humans the nobility of sorrow, pain, and joy.