Hey guys! Im doing this guide to help you decide which upgrades to get first, but i'd like to reminds you that it's totally based on my experience with the game and what i consider to be the best, may not be for others. Also, the order is based on which move i mostly used to beat the game on the harder difficulty.
Since you don't have unlocked all the skills to upgrade when you beggin the game, you probably won't be able to upgrade them in the order that i'll show you. Just keep in mind what is best, get what is available in the moment and save Karma to when you unlock others skills.

Probably the most broken skill from the game, because it allows you to slow the time inside the bomb range and the enemies will not be able to move. Along with the "Perfect Charge" skill, it will be your main strategy at the hardest difficulty of the game, since you will be able to do an almost infinity loop using both. Check out the vid below:

Simply starting the battles with 1 Rot Action will give you a big advantage to use whatever is best for the situation.

Something the game doesn't tell you that make this skill even more powerfull, is that when you perform the perfect timing, along with more damage you will also receive more Courage charge for your Rot Actions and that's why you can do the loop with "Rot Infused Bomb".

Debuffs enemies defense with your bombs (that will be your main strategy), think i dont need to say more about it.

Might look useless at the first moment but its really useful to buy you some time to take a look at the position of the enemies on battle and also to aim more accurately. The game doesn't tell you, but this skill has a cooldown (5 sec) and to be more confortable to use it, go to Options > Accessibility and set "Automatically Focus Aimed Bow" ON. Doing it you won't need to press another key to use focus.
You can also "use Focus" by aiming while jumping (this don't require to have the skill and don't have cooldown).

Extra bomb for your loop or when you misused the first one. Extra tip about bombs: you can 1HKO the Moth enemies by throwing a bomb on them and exploding with your pulse; also all the orbs attacks from Mage enemies can also be stopped by throwing a bomb on them and pulsing.

Since both skills improves the amount of arrows i put them together. The bow might be underestimated due its low damage, but its really useful. As mentioned before, you will use your bow to gain some field advantage with the "Focus" skill and also for ranged attacks. Even though the damage is low, the main point for shotting in battles is to fill your Courage meter to use Rot Actions, so don't mind running and shooting a bit.

Since both skills improves the shield bar i put them together. Even if you rather to evade or parry the enemies instead of simply shielding, this skill is a must since you will use your shield bar to perform the "Dash" ability. The "Dash" its incredible poweful, quick, can break enemies attacks, avoid some attacks and also grants a good Courage charge, so don't underestimate it like i did when i got it.

In my opinion the parry window on the game is a little bit tight, so i rather not waste my time mastering this resource. But the "Parry Courage" is a powerful skill if you are a player that likes to master parry and use it all the time, since perfect parrying will provide one Rot Action.

I know that people that are on their first playthrough may think that i am mad to not list Rot Hammer and Rot Infused Arrow before, but heres my point:

Rot Hammer: massive damage and have great area, but its really useful only when in the lower difficulties since on harder you will want to priorize whatever provides you any protection/survival because you will die in 2 or 3 hits.
Rot Infused Arrow: the damage is not that good but the piercing is useful sometimes. You have better options than this, so you will barely remember that this skill exists. In my opinion it's more useful on shooting challenges than in battles.

Well, after getting the above skills feel free to get whatever please you. I hope this guide helps you to enjoy this incredible journey with Kena!

PS: I got the Rot Level and Karma required informations from a website[kena-bridge-of-spirits.fandom.com], so i cant guarantee that it's 100% correct and if it's something wrong feel free to comment and i'll fix it in the guide.