Here you can definitively see a difference in both characters, both in the fact that they have evolved, aaand... well... It's story time!
One day, a tribe of Persian found little Kenko alone, when he was still a Shinx, and the leader lunged at him, scratching the eye that is now covered under his tuft of hair. After coming back home to run into his parents' arms, he was rejected, as the only worthy characteristic of the Shinx line was to be able to see in the dark, and having an one-eyed member in the tribe was useless. In short, his tribe, including his own parents, rejected him.
Having nowhere to go, little Kenko begged his friend Maru for help, and the rodent tribe warmly welcomed him.
Much to their sorrow, the next morning Kenko woke up to everyone but him and Maru gone. What was worse, Maru was bleeding from his cracked cheeks and crying. The poor Pichu told his friend that early in the morning, the Persian tribe had ambushed everyone and that he had used his own Electric powers to save his family. In the end, he forced his cheeks to send too much electricity and ended up hurting and inhabiliting his main source of power, to no avail unable to save his family.
From that day on, Kenkodenai and Maru decided to set on a journey far away, to find the Persian tribe and avenge Maru's family. But most importantly... while Maru had decided to fully evolve to become stronger, Kenkodenai refused to evolve into Luxray -- he didn't want to become the species that had rejected him, and so, sought to demonstrate that he didn't need to fully evolve to reveal his full potential.
Maru keeps his smile despite what happened, only striving for hope and for things to get better, always up for adventure. Kenkodenai, on the other hand... is stoic and reserved, but deeply trusts in his best friend.