Here's the idea:

The main character is a male, unicorn/earth pony magician (haven't decided) who is able to mesmerize ponies through a deck of playing cards; the cards he uses depends on which purpose he has for them/what he wants them to do. He's a bit of a jerk with a heart of gold - a bit of a showoff in public, though genuinely nice to the audience and more relaxed when not actually on-stage or in-the-spotlight, so a bit of a public persona. He's also a little bit of a coward.

The magician is threatened by a crime syndicate/mob looking for more territory. Using his abilities of deception, mesmerization (not sure if that's a word... ah, well), and sleight of hand, he takes out the main members of the mob with the help of a mare, either one of the Mane 6 (less likely) or a Background Pony (more likely). His reason for going against the mob is the threatening of his livelihood, extortion, and general bad things done by the mob taking over his town, all with a chance of showing off his skills, though it becomes more about helping out other ponies than it is just about his talent for magic.

What do you think? And should the other character be one of the Mane 6 or a Background Pony?

When dealing with a Crime Syndicate, the best place for them is in one of the larger cities such as Manehattan. As such, it's unlikely (but not impossible) to run into just one of the Mane 6 alone, so a background pony would be a better choice.

Suggestion: Depending on how much this character is a coward (and if you want him to develop into a better person) a better idea for getting involved with the mob is by accident than the mob trying to get more territory. Such as another magician setting up shop where this main character does his act. The main Character finds the act this newcomer is doing to be insulting, and challenges him. The events lead to the Main character finding out the new magician is a fraud and his traveling act a front for moving goods for the mob. With his interfering, the mob now takes an interested in the main character and wish to hire him to replace the fake they put up front. Using his skills to save his own neck at the time, the background pony in question can end up showing the main character just how skillful he really is and how he isn't such a bad person as he believes, as the two face against the mob.