Trevenant has been with you for a long time. You first met your pokémon as a flimsy Phantump, a lost and frightened little thing, wandering aimlessly thru the forest. It only ever seemed to find comfort in your company and when you proposed to it to become your first pokemon it didn't object.

The following years have been good to the both of you, you steadily rose above the ranks of the pokemon league and eventually maneged to become the new ghost type gym leader of your region. Phantump evolved shortly after, and it kept being your closest friend even as a Trevenant.