Daiku no Gen-San: Robot Teikoku no Yabou

From the series of "Hammerin' Harry". The second Game Boy game takes out heroes to the final frontier - outer space. The basic gameplay is much the same as its predecessor, although it's a bit more difficult. At a few points, you'll be accompanied by some robot helpers, who show up at specific points in each stage to spring you over walls or supply extra firepower. It also has a particularly cool final battle where the roles are reversed - instead of being a lone character against a huge machine, you control a huge machine against one single character, a wily ninja who blinks in and out of existence. This one was only released in Japan.