>Sound effects for some actions would be good. Some actions missed them and it felt incomplete. I can't test it now as I'm at work but I believe there were no effets for reflected bullets/hurt animations? Due to that, it was hard to notice your bullets were being reflected and were hitting you.
>There should be an invincible moment after taking multiple hits in a row. Right now, every damage stacks up quickly, which can be really unfair.
>Sideways moving is really slow. Adding actions such as rolling/sliding could make gameplay more lively.
>A way to 'control' your friend would be nice, as this way he wouldn't be that useless at some enemies. For example, his attacks could be assigned to a button and act like secondary weapon, much like Gradius' missile button. This way the player can plan when to use it, instead of being activated at a fixed time.

The music was better this time around, compared to the MID files you sent me of Bouncy Cat. Still, some of the tracks were still weird and dissonant. I don't know how to teach you composing, as not even I know how to compose. Still, I'll try my best explaining it. Please note that I am NOT a musician, thus I don't know the proper terms for some of the effects or techniques.

It's good to work on your instrumentation. Some tracks were filled of 'twangy' synth leads and while they aren't bad if used moderately, they get kinda boring after a long time. Imagine your music as an orchestra: there are several instruments to choose from, don't be limited to a handful of instruments only. Another way to make it better is to rewrite the chorus part with different instruments to give it a different flavor and making the loop bigger.

Also, the lack of harmony in the melody is kinda off-putting. As you might know, some notes combine better with others. You can use this note combination to create interesting 'bridges' and 'chorus' within the music. For starters, notes that are too close together tend to sound bad while notes that are apart tend to sound good. It's good to have a music keyboard to play those notes when writing music (sometimes I use my Melodica to playtest the notes before writing them in the program), this way you'll save time when composing.

Another musical tip is to set the bassline first. While this isn't obligatory, writing it first helps setting the mood of the entire song. With a physical music keyboard, you can find a good bassline more easily, and you'll be able to test chords. Chord is a group of two or more notes that play together. If the chords don't combine with the bassline, then you'll have a dissonant music and might not be what you intended.

Finally, I'll say that there isn't a right way of composing, at least, not with my method. I simply hum and playtest my songs and write down what sounds good, so it's hard for me to explain it in a concise way. What helped me, though, was to cover some songs. Covering songs is good for practicing, as they have a well-formed chord/bassline progression which you can use in your future original works. You'll notice that a lot of songs tend to group the same notes together (for example, C (do) with E (mi) (which is part of the C Major chord)), and composing will be more logical and faster.