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Kai and Zero, when I first saw it in screens, immediately reminded me of the legacy of 2-D Zelda adventure games. The game (and the producer himself, in our interview) did little to dissuade that connection. There are certainly several similarities between Kai and Zero and most Zelda series entries, probably the most obvious being the "guard boosts" that Kai will obtain along the way—they're round heart containers, basically. And, true to form, the game's worlds consist largely of environmental obstacles that will require you to use things you've picked up along the way in order to progress.
Those things are the Omega Elebits, a collection of several special little creatures, each possessing some sort of power that is generally useful in traversing the worlds. The basic elementals make an appeareance, of course, but there are also some others with powers of magnetism, speed, or even water absorption. While you'll often find uses for newly-acquired Omegas in the immediate world, you'll also continue to use them in later worlds as well; they're not one-off "keys", which is very nice to see. But these guys need power to operate—power you'll gather, throughout the game, by looking under rocks, shaking trees, and being quick on the draw to capture the other Elebits that inhabit the world, converting them to watts in order to get your newfound abilities moving.
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In contrast to many modern games, Kai and Zero's level design can be a bit of a throwback to the light-on-text days of the SNES; rarely will you find signs sticking out of the ground telling you how to solve a particular environmental puzzle; much like hide-and-seeking Elebits, you must rely on your powers of observation and be ready to try various things to get to where you need to go. This is potentially a great blessing for those who like challenge in their games, but it's moderated somewhat by the occasional WTF moment where you're struck trying every Omega in your toolbox and, once the solution is found, end up wondering how that could have made sense in the first place. (Or worse: At one point, I was convinced I didn't have the necessary Omega—a visual cue suggested one that didn't actually exist in the game—and spent a pretty ridiculous amount of time wandering one world looking for it.)
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Outside of what you need to get the main quest done and dusted, there's plenty of extra content should you be so inclined to pursue it. Each world has hidden pink Elebits that, when all three are found, will yield up a spare guard boost. There are battery upgrades which may be in rather out-of-the-way places. There are even optional, completely new Omega Elebits that you can get by going back to the various worlds and checking out previously-inaccessible areas. (I didn't do these before I completed the game, but it's my understanding you can.)
Kai and Zero ends up a solid game that's rooted firmly in the classic 2-D adventures of yore, and it turns out to be just what the doctor ordered for the homesickness I didn't know I had for games of this type. It was particularly nice to be able to play a new game in this genre with its own fresh take, even if there were some minor problems here and there. If nothing else, it proves that even as we pursue new types of game machines with new input possibilities, there are still new depths to be explored—even in the classics.