Black Cat - Set in a fictional universe, Train Heartnet, once an assassin for Chronos, an organization bent on world peace that rules one third of the world's economy, is now an easygoing Sweeper (licensed bounty hunter) traveling with his partner Sven Vollfied.

Reiji Ozora is a chronic quitter who never completes what's assigned or handed to him, but one day Maiko, his closest friend, walks him to a local arcade where the latest game craze is happening: Dragon Drive. After meeting Chibi, a seemingly meek dragon, Reiji wants to make himself and Chibi strong

OMG BLACK CAT!!!! i read this in high school!