Atlus is a japanese video game company, who also have a US branch. Atlus Japan are the creators of the Shin Megami Tensei series, which has a cult-like following among some console-RPG fans. They are also largely responsible for the Ogre Battle series (Ogre Battle, Tactics Ogre, Ogre Battle 64, maybe more). Atlus USA mostly (or entirely--I'm not sure) republishes Japanese games for the US market (including translation, localization, etc).