My background:

I come from league, played top/jg at a diamond lvl in my peak, been playing pred for a few months, and very quickly became a Kallari one trick pony. I have 200 games on her, and I like to tinker - tried lots of different builds and strategies.

Goal:

Kallari is one of the strongest and most fun to play heroes in the game, but she stays pretty consistently at the lowest win rate of any hero.
My goal is to bridge that skill gap and make her more accessible to everyone.
I will put a TLDR at the beginning of each section in bold for anyone looking for basic direction, and then a deep dive of each part below.

Strengths:

High Burst, Stealth, High Mobility, Counterjungling, Objective Control
If you wanna delete someone in 2 seconds, you came to the right place. Kallari can take out key targets faster and better than any other hero. Here’s a quick 60s video showing this:
https://drive.google.com/file/d/1TtBI9CvM3rKCiY0pUKNJheCaJGeFbRQz/view?usp=sharing
Doesn’t matter if the enemy sparrow is 1-0 or if she is 30-0 - if you get close to her undetected, she’s still super dead. This capability can swing a game from certain loss to winnable.
Your mobility and stealth doesn’t just help for assassinating, it helps you get out of a bad spot. Understanding this is critical in maximizing her value: Kallari can do aggressive / risky things with less risk. Like counterjungling, split pushing, soloing fangtooth, stealing objectives – if shit gets too hot, just pop stealth and triple jump over a wall.

Weaknesses:

Squishy, Weak to CC, Skillshot dependent, CD dependent
When you do get caught in a root or stun, you’re probably dead. Kallari is squishy. Even in a fair fight 1v1 trading damage with an adc you can die. Your damage is heavily dependent on landing your daggers, and ult, which are skill shots. Your CDs are pretty long, and when they’re down, you are pretty useless. So don’t fight fair. Sneak behind someone, blow all your CDs, and get out. If you need to reengage, wait til your CDs are back up. Don’t stick around, exposed, trading auto attacks.

Skill Order:

Daggers < E < Q, with a small caveat. Lvl 1 Q sucks. Q heals you. Getting it to lvl 2 early gives you more early healing and lets you keep farming in situations where youd normally have to back. Lvl 2 Q helps you assassinate too by giving you longer stealth time. Otherwise, max daggers. Daggers are your bread and butter for farming.

Items:

Ward Sweeper - Nex –Malady – Envy – Perforator - Mesmer – Nightfall – Painweaver
Ward sweeper is important for a few reasons. First, because you are an ambush predator, if they know you’re coming, it might not even be worth ganking. Second, even just the act of sweeping wards on the map will give your laners pressure. The enemy laners should be worried the whole laning phase thinking, “where the f%#$ is Kallari??”. Lastly, theres no control wards in this game, so the only way to control objectives currently is to have a sweeper on your team, and you’re the best one to have it.
Okay, now to the item build. Why physical pen? Building more and more armor has diminishing returns – that means that building more and more flat physical pen has accelerating returns. Here’s a graph illustrating how much damage goes through, using an ADC’s armor as a baseline:
https://drive.google.com/file/d/1qko61fyjnIIrc2kd6RMnx2S9LyKQ-YTq/view?usp=sharing
Also, envy already gives you 100% crit, for free, for 2 auto attacks. So buying more is a waste. I rarely auto attack more than twice in a rotation (Can see this in beginning video).

Combos

Melee engage: Close(Q) – E – AA – Dagger – AA – Dagger – AA – (Q) – AA – Nex – R.
Ranged engage: (Q) - E – Dagger – R – AA – Dagger – AA – Nex
In general, there is a close-up style combo if you’re able to get close/behind someone, and a combo if you need to close some distance. The beginning video shows variations of both.
The close-up style combo is the max damage, but the tradeoff is its harder to get into position. The distance combo does slightly less damage, but has less positioning challenges. The most important thing is to ambush them – optimize for this instead of damage. Never walk up straight in front of someone. They should already be half dead by the time they even realize you are on them or whats going on. I find myself doing the distance combo more often, especially against an adc, where even the lesser damage is enough to kill them. If it’s a tankier hero you will have to take on more risk of getting closer.
General combo notes:
E increases your dmg against the target, don’t ult or use other big damage before your mark is active
Envy makes your auto atk 100% crit after your ult AND after exiting stealth (you must actually enter stealth, you cant just Q-AA). Nex and triple jump don’t proc envy crit.
Nex stores 20% of your damage as burst, so use it towards the end
If someones backing under turret under 25% hp, E-dagger-dagger will kill them. Gotten soo many kills like this.

Early Game:

Steal red buff, focus on counter jungling and farming, solo fangtooth, only gank pre-6 if opportunity presents itself – don’t look for / force ganks

Early game is important so I’m gonna write more than other sections: Kallari is the best counter jungler in the game due to her ability to blatantly over-extend into enemy territory with little to no consequences. I’ve found more consistent success in the early game spending by time shutting down the enemy jungler than I have trying to gank early. Gank success is heavily dependent on your laners, and you can’t control your laners, but you can definitely control the enemy jungler. Steal their buffs. Steal their camps. Then, go clear your whole jungle. After that, their red should be up again, go try to steal it again. Why not? Steal their blue again too while you’re at it. While you’re doing your thing, if you walk by a lane that is favorable for a gank, go for it. But don’t actively look for ganks, or force them, or wait around for them. Your mission pre lvl 6 is to farm and take the other jungler out of the game.
You can also solo fangtooth pretty easily at level 4. Sweep the area, and just start it. To be extra damage efficient, only use your daggers once your E is up. If someone comes or things start to go sideways, you can just jump out of the pit. Other junglers don’t get to take this risk unless they want to flash out of the pit when things go bad.
Video of how to red steal: https://drive.google.com/file/d/1GNW-lS84OmAWrsyEYPNdfVYlYCcpDpCS/view?usp=sharing

Mid Game:

Help strongest laner get fed, get their turret down so they can roam, keep soloing fangtooth
Your goal in the mid game is to unleash your strongest laner from their lane so they can start pushing turrets and objectives on the map. Don’t fall into the trap of trying to save your 0-3 greystone who is spamming pings. Go gank for your 4-0 carry, help them get their turrets down & can start roaming.

Late Game:

Gonna write even less here since every late game landscape is pretty different and what you do is going to be more up to your macro than anything. Just know that two good tools you have in your toolbox are 1) you can one shot someone and 2) you can split push really well. General assassin teamfight tip, wait til the chaos of the fight starts before you expose yourself. If their carry is strong, go for them, if yours is strong, protect them.