Katana should be from the "hidden" room that Izumi left open during Dinosaur hunt. Izumi chose to be set free without memories with his 100 points, so he shouldn't have any powerful weapon.
See what I mean? He doesn't even have to follow the law of equivalent exchange when transforming. If kurono hadn't come in with that sword Reika and the other guy would've been dead. On a related note when the hell did kurono learn how to use the sword?