This is fun so far! The game feels nice and poppy and I like the various effects when you hit an enemy.
The orb is a really cool mechanic, but I'd like to see it expanded more. So far I've gotten to ~70 kills and the orb only started to become a threat past 60 or so kills. It would be nice if it grew more quickly or moved faster, so it's more of a threat even early on. It might also be cool if it interacts in some way with living enemies, like making it so if the orb hits an enemy, the enemy becomes more powerful. Basically, play up the importance of the orb.
I'd also say add more enemy types.
You might also want to add some feedback effects for when the orb absorbs dead enemies. Like maybe a little concentric circle closing in on the orb, or at least a small sound effect.
(I had a thing about there not being sound effects here before, but it turns out I had something plugged into my speaker's auxiliary port, so I just wasn't getting audio from my computer )