Solid take on the VS genre. It controls well and the art is cute. I played as Xelvy and I agree with @qwerty741 that the upgrades feel off for her - you're just looking for bombs + stats, none of the spells make a difference in her gameplay.
I liked the high impact of upgrades but the game feels impossible to lose in its current state. Things I'd like to see:
• General difficulty increase
• More interesting attack/mob patterns
• Something unique to set it apart from other games in the genre
I'm sure you've played lots of games like this one, but if you need some inspiration, I highly recommend checking out Death Must Die and 20 Minutes Till Dawn. They both bring some cool mechanics to the table that make you want to keep playing.