Isulven

Isulven, aka Longditto is a member of the First Mile and part of the trio formed by itself, Kenkachu and Mortus.

Capture and Evolution
The requirements to obtain an Isulven seems rediculous at first glance: "Capture a shiny Ditto and fight battles with it in a cold area until it evolves into Isulee, then level it up until it evolves into Isulven." But this is made relatively simple by a sidequest late in the game.

After the trainer earns the trust of the people in Sheol by defeating their gym leader; Horus, they allow the player to enter a hidden portion of a cave where they can reach a room with a high Ditto encounter rate. The shiny/normal color ratio is also reversed in this room, making it very easy to catch a shiny Ditto (and very rare to find a normal-colored one)

With the shiny Ditto in hands, all that is left to do is evolving it in cold climate. (precision needed)

Isulven in Battle
While it is the evolved form of Ditto, Isulven really has nothing to do with Ditto as far as combat is concerned.

The Pokemon is a special wall with relatively low attack power and high speed. Isulven's signature move also let it make itself immune to special attacks while doubling physical damage against it, effectively increasing it special walling potential. Isulven's stats are good special attack, low attack, bellow average physical defense, very high special defense and high speed.

Isulven also wield an ice-type variant of Plasma Slash which requires Tail Wind to be in effect to be useable.

(More information will be added later, Isulven has a very detailed movepool and stats spread but it is currently lost in the Pokemon Plasma archives. Details will be added as soon as available, sorry for the inconveniences.)

Trivia

 * Traceur Lee is the only trainer in Celvice to own an Isulven.
 * Isulven is one of Zee's personal favorite Pokemon.