All characters have a mandatory set of upgrades that need to be done to take your character to the next level. These are the four upgrades connected in a line from one level upgrade to the next one.
The other three upgrades are optional but, if you decide to upgrade them, this will make you stronger even if you're at a lower level.
It all depends on your strategy, Zoobster! If you want to evolve more with weapon skills, it might be better to make sure to purchase all the secondary upgrades, but if you just want more damage and life, concentrate on making all the four primary upgrades to get to the next level faster.
Also, do not forget to collect the bonus emotes, items, and skins you find along the way! Have fun!