It could be a pony who is extremely extroverted if Twilight is introverted. Someone who loves chaos and spontaneity instead of Twilight's love for order and planning.
In this kind of fanfiction, Twilight Sparkle might face a character who has completely opposite traits. For example, if Twilight is studious and cautious, her opposite could be reckless and carefree. They could initially clash due to their differences. Maybe the opposite character challenges Twilight's way of thinking and doing things, which forces Twilight to re - evaluate herself and her beliefs. It could also lead to some comical situations as they try to interact with each other in a world where their behaviors seem so at odds.