Bio Motor Unitron serves as the Neo Geo Pocket's answer to games like Pokemon and Monster Rancher. Like Pokemon, there's lots of things that can be collected and the battles are strictly menu-based, but like Monster Rancher, most of its key battles take place inside an arena.
There's a bit of backstory to the game, but its really easy to overlook. Suffice to say, you pilot a big robot called a Unitron. Your Unitron can be equipped with different arms, legs, backpacks, and other parts. Most parts are purchased in stores or won in dungeon battles, but the really good arms (which serve as your weapons) must be developed by your engineer. Arm development is done by taking an existing arm, applying materials like steel, gold, silver, or tungsten to it, and hoping that your engineer's level is high enough to successfully upgrade the arm. You can also throw various items into the mix to help increase the chances of success. The more powerful arms are made from extremely rare materials, which you must find or win in one of the game's four dungeons. There's also an elemental twist to the game. Each arm and Unitron has an elemental affinity - so fire-based arms work well againt wood-based monsters, but not so well against water. This gives the game a rock-paper-scissors mentality, as you must rotate your weaponry regularly to combat different types of foes.
Arena battles make up a great deal of the game. As in Monster Rancher, you'll start out as an E-class fighter, hoping to make your way up to the coveted S-class. But there are also four randomly generated dungeons, each with its own end boss monster, which will give you a special item if you succeed in defeating it. While you can get through most of the arena battles by simply hanging out in dungeons and leveling up until you're strong enough to return to the arena, these special boss items really help out in the more advanced arena battles.
The real fun in the game is experimenting with different arms and materials, hoping to discover a combination that yields a rare, powerful weapon. While not the deepest, most story-laden RPG in the world, Bio Motor Unitron makes a really good portable RPG, and lends itself reasonably well to being played in short bursts as well as longer sessions, once you've gotten addicted to finding new items and materials. Definitely an A-list, must-own title for anyone that isn't completely opposed to menu-based battling.