Pretty cool game! I quite like the whole coop nature of the gameplay where you are two characters helping each other out by either splitting up or combining your strengths. I especially loved the tutorial sequence where the gameplay and story are intertwined with the narrator simultaneously teaching you the game while introducing the characters and lore: charming stuff. Definitely my favorite part was the boss fight: felt like it had a good variation of attacks that were fun and satisfying to battle through. The gameplay did feel a little repetitive at times and could use some more interesting set-pieces and new mechanics to vary it up, but I had a great time and would love to see the continuation of this and see how it develops.
The big downside for me was the lack of sound effects! There was such a lack of sound in this game that it was always incredibly jarring when it would show up, like how it plays a sound when the boss slams down but not for its various other attacks. I'd almost suggest that the game should just have no sound at all and only have music instead if you're going to be that lacking, but ideally I would say you should have sound effects more prominently for the various actions you and the enemies can take (movement, jumping, attacks, etc). Other issues were some minor things like controls where I would have difficulty getting a rising lumi (super jump) to go off consistently despite my best efforts, and that the combat could be simultaneously too hard (difficult to dodge the waves of bullets from normal enemies, and way too difficult to dodge the boss when it spins multiple times) and too easy (why dodge bullets when you can easily swing your sword to stun-lock all enemies from being able to attack).