A powerful being imbedded with energy from the mysterious Chaos Crystals ( The Sonic Boom universe's take on Chaos Emeralds ), Shadow's past is mostly unkown to everyone besides him, with others thinking he's just some freak that hangs out in a cave & broods his way through life.
One thing this Shadow has in common with his Prime counterpart is an intense competitive streak, always up for a challenge of strength & ability when it concerns him most. He also has quite the sketchy history with Sonic, holding a passionate grudge against him nowadays & never wasting an opportunity to battle him to prove superiority.
This Shadow is riddled with feelings of being betrayed in the past, driving him down a vengeful path of villainous cruelty & a lack of care for most things around him. He's willingly endangered his own universe by trying to create a paradox, & will often leave any hero work to Sonic & his friends while he goes his own way. Clearly the only justice he even remotely cares for is his own. And so long as he can get Sonic to pay for what he's done to him in the past, this Shadow is willing to go along with whatever evil scheme necessary.