♊Gemini, the mutable air sign, represented by the twins.

If Shadow's birthday is June 19, that would make him a Gemini. Gemini's strength is adaptability and quick thinking. 

Geminis don't have "evil twins" or split personalities in their heads or anything, the twins represent how Gemini is often of "two minds" thanks to being characterized by fluctuating (mutable) mental (air) energy. Rather than a "good" twin and an "evil" twin, it's more like a "vanilla is my favorite!!" twin and a "no, chocolate is my favorite!!" twin. Also, the stars you were born under arent going to give you a split personality, but they might give you stunted magicka and spell absorption 50% on self etc etc
But i drew shadow with his "evil twin" anyway pfft

In this picture, Shadow represents a more logical, disciplined side, and Mephiles a more emotional, impulsive side. Not really good or evil. 
Mainly I like flippy opposite pictures, so this image came to mind and I thought it'd be cool lol.