We like the character design!
The idea of shifting between characters, and that one of the characters power up the other one is a really neat idea! It takes a while to get used to, but when one does it's easier to focus on the gameplay.
One of us has played a lot of shmups, and thinks this type of gameplay with that big of a shield could be good for introducing new players to the shmup genre - as it doesn't require as much precision as the more hardcore type of shmups.
Our feedback on improvements is that we'd like to see a bit more juicyness in the game. A slight sense of acceleration, particle effects, some animations or just simply scaling the sprites up and down slightly - we think there's ways to really breathe some life into the game with fairly small means.
Overall an approachable shmup, well done!