First off, the game does not allow ordering within tiers, so I'll do my best to explain when I believe a character is either towards the top or bottom of their respective tier.

But yes, Here is my tier list:

For me, an S-Tier character needs to be flexible and top tier in nearly any meta. As these are the characters furthest out of line, I'm going to talk about all of them individually.

Elemi (Wannabe Lucian)

Elemi is the character I would comfortably place to the bottom of S Tier, though she squeaks in in my opinion. This is in large part because of how insanely dominant she is in the early career progression, when she is by far the best marksman available and when the game is at its hardest due to limited champ pools and items, which a player is likely to just do better than the current AI.

Elemi is the best tower pushing bot laner in the game and in dragon fight centric metas her value goes up a lot due to strong early damage. In addition, she synergises very well with Peniang Zhu (Bird face Support), who I have placed S tier as well. Bird Face is good enough that being a good pairing is enough to boost Elemi's placement a bit as well as even if you choose not to take one of them yourself, Paisai is the other best early career support (and might be better than Bird face in the early carerer actually) and also pairs very well with Elemi. As such, Elemi becomes super high priority in these games.

Elemi remains good throughout the career, largely due to some very good item synergies, though Deep Space is a very strong later career threat and well, nothing compares to Digo. That guy just completely breaks the balance the game is built on.

Justice (Wannabe Garen)

Justice might well be the best character in the game.

This is going to be a hot take to some and the other tier lists I looked at on this game certainly didn't agree, though I am willing to argue this point strongly.

First of all, Justice's ultimate spell... It's pretty bad. Super situational and even in those situations it's normally not worth using as it uses her passive stacks.

So. Why is Justice so good then? Her passive and the way that combat works in this game.
Justice's passive is an execute that triggers on any enemy below 25% health when her passive is stacked, even outside of her turn order. This passive can be stacked by casting her cheap spell, which also pushes towers insanely fast or by applying her shield, which makes her stronger for the next fight and very tanky. This is good because you want her to stick around. In addition to this, Justice is a "Hybrid" hero and can therefore buy tank items when they're strong in the meta, physical items when they're strong in the meta, AND buy strong magical items when they're in the meta.

This all sounds pretty simple, and it is - but what really makes Justice so strong is how combat plays out. Characters target each other randomly, based on presence or influenced by passives. Justice's passive execute lets you bypass the enemy marksman living on 20 hp for 7 turns and wiping the fight. It lets you cut Frank's massive HP pool in half. It lets you get to Kid before he finishes an ally and heals again. Most of all, this can't be dodged. Reflex/Mechanics hell get ignored by Justice completely. Super consistent, super safe, super strong.
Justice beats her "counters" like Charon because she triggers the passive early.
Justice destroys squishy compositions because when one dies, it's that much easier for the next to as targeting RNG is reduced.
Justice pairs extremely well with assassins that target carries, or AOE like Niels. Though Justice doesn't need these synergies, they can elevate her further.
Justice dominates early objective fights, especially ones where just one kill is needed.

And, this is going to be a theme you'll see with a lot of the characters I've rated A tier because honestly, it's enough to get that far alone the way this game works: Justice is a tanky character that deals damage.

Hass (That character everyone agrees is S tier)

Another tanky character that deals damage. Also arguably strongest in the game, definitely competitive at the least.

There isn't much to say on Hass really, he just does what he says on the tin. He's a stat stick but he's way overtuned. He is supposed to be a buff stealer, though this aspect can be completely ignored because of how strong he is and he can be first picked anyways.

Shadow Blade can give Hass upwards of 1600 gold in one spell in the late game.

Shadow Imitator copies part of an opponent's stats - this includes armour and includes out of combat or temporary buffs that are on the opponent.

Not much more to say here: Pairs very well with Beverly and Blocker as they let him cast his broken Shadow Blade more and Beverly amps his stats even further. I did a playthrough where I played a hard carry top team and getting Hass on a carry player was always easy mode.

Digo (The character that breaks the game.)

The third of four characters I think are the four best in the game.

Digo completely breaks the game. Completely. Digo takes 0 damage unless all other team mates are dead. This, in concept, should balance itself as on the up side the carry can't get randomly sniped but the other team mates get focused down more and playing tanks means less damage from those sources.

If played like this, Digo's still really good because he ignores armour completely, but that's not why Digo is so broken. Digo is so broken because some of the other characters who are the strongest i the game are his best possible partners:
Hass, Justice, Wolfgang, Zealot, Cubey, Frank, Babe, Kid... You get the picture. Tanky, safe solo laners who also deal damage. All of a sudden, you're just winning. There are too many of these to have your options banned out so if I'm red side I will ban Digo.. If the enemy even plays it. If I am blue, I will first pick it. There are very, very few exceptions to this rule as soon as Digo becomes playable.

His big initiate is really good too but honestly, I personally don't value initiation spells that highly in this game. His is one of the few I use though.

Beverly

Very simple really: Take a scaling carry like Cubey or Hass, pair them with Beverly, they get even more insane.

Beverly is also the one exception that can make Qube, whom I placed in C tier, a top tier pick. It was hard to play a damage focused comp through a top carry but Qube made this possible with Beverly and actually got really insane with Dylan mid. Dylan + Beverly are a very strong combo if your top laner is strong.

Zealot

Extremely meta warping. Early career pick/bans are solely centred around Zealot, Elemi and their counters/synergies.

Over the course of the game, more counters to Zealot have been released so he is not as broken as he once was but that's what drafting is for. Get Zealot into a matchup where the enemy has no AOE or on-attack debuffs and your odds are extremely good. Has very good ganking and counterjungling. Junglers are the main place I value initiation abilities, though counterjungling, especially with the stacking counter jungle trait (Opportunism) is extremely, extremely strong and cuts variance hard. Zealot is very strong on most junglers, though gets outperformed by some others in these sorts of strategies due to the 75% power penalty on levels and gold.

IMO, Zealot is the best way to make a weak Jungler a carry but not [that/i] insane on a strong jungler, though this is very much a hot take. Regardless, Zealot always needs to be a pick/ban consideration and is a very easy S tier.

Cubey

Everything that makes Frank good dialed up to 11 with consistency and insane synergy with Blocker and Beverly.

Fairly balanced without synergy from one of the two but still consistent. It's when Cubey is paired with one of them that it is just not okay. This is how you make a 20k HP, 1600 damage character.

Peiniang Zhu (Bird Face)

Bird Face is a very easy character to underrate and on my first couple of playthroughs I thought of Bird face as good but I rated stuff like Beverly, Paisai and Dylan so high that I struggled to justify playing them. In my opinion, Bird Face is very similar to Paisai early on in the career but extremely overpowered in the end game once the level based scaling on the passive effects are gained. Bird Face is a late game scaling support that frequently outdamages any other member of your team, especially when paired with AOE and works very well with slower compositions or against multi-acting characters (like Zealot) or Felyn (the weakest marksman in the game IMO but the AI keep picking her for some godforsaken reason and handing games for free).

As the game goes on, Bird Face gets more stacks on Thundercloud by casting it throughout the game. A scaling/stall comp with Bird Face can often go down 10k gold and still win the final fight, all with the support as the game's 40% damage dealer and no offensive items.

Bird Face however, has some very real counters. Do not pick them into Wukong or Fatty White. Tivie is a counter on paper but she's too weak to bother not picking this character blind if that's the only counter the opponent has, you're still getting the better trade in the vast majority of cases.

The best A tiers are Niels, Deep Space, Dylan, Wolfgang and Babe in my opinion.

The worst (but still good) are Charon and Peter.

Aurelio and Xiangxi Ke

In my opinion, these are the two best 'carry' junglers in the game. Both have strong counter jungling, though Aurelio's is better. Which to play largely centers around team compositions in an individual game.

Niels

I struggled on whether or not to place Niels in S, A or even B. It's close and difficult but in the end I felt top of A was the best fit. Niels is very, very strong in his niche. Ultimately, I think the reason is so good is because of how well he pairs with Justice in particular, counters Zealot and is good with other strong picks like Xiangxi Ke for picking off priority targets.

Frank

I'm sure some are surprised Frank wasn't rated higher here. I think he's good: but he's also pray to some of the strongest characters in the game, with a poor matchup into Hass, Justice, Wolfgang, etc. Playing a Frank-centred strategy is completely viable and strong, it's just going to require some bans to make work and can be brute forced through in a way that I think is convincing as those I placed in S often cannot in their niche.

One of the biggest advantages to Frank is that because he has 0 armour and health instead, he matches very well into Digo. Also matches well into Elemi due to double shot but again, that's pretty possible to brute force as it's still high damage.

Wolfgang

Preys on slow teams. A very good defensive solo laner. I have a huge bias towards characters like Justice and Wolfgang who can go either top or mid, slowly scale and are very difficult for the enemy to gank and kill. They make scaling very consistent.

The best B tiers are: Blocker, Wukong, Miki.

The worst are: Fatty White, Gang, Gillis.

Gang

This guy has a niche, though his single target damage is unimpressive. The problem is that his niche is doing large AOE damage, which is most efficient when trying to kill squishies and when there are more squishies there's more out there to kill him quickly. He's countered by what is in my opinion his best matchup.

It's fair to argue that AOE is always good, and is good against many tanks due to the overall output, but his damage is just not that high. It's piddling even with the AOE compared to Deep Space's single target, Elemi's double taps or even Digo's defence ignore. He can kill people outside of combat and try snowball though to be fair.

Miki

Very, very meta dependent. When the first dragon's reward is too strong, he's one of the best in the game. In most metas, he ranges from average to pointless when an enemy can just forfeit the dragons for minimal loss.

Gillis

Gillis has some very, very strong tools. The problem is that her passive is actively detrimental. She would be better off without it because of how random targeting can be, there's just too much variance to justify running her over the alternatives. I feel bad putting her here, because there are metas where she's very good. On average though, she's middling at best.

I can add to this if there's interest I guess, though for the most part I just think they're undertuned. I don't have a good read on Kaka though, he's one of the few characters I haven't played, though the AI has not impressed me with him.

Felyn

I feel like this character stands out from the others. Being able to push towers as aggressively as she can is a very good thing in the right circumstances and I am sure there are metas where she's good. It is entirely possible I rate her low as Hell mode often has your players with worse stats than the opponents. Felyn is, at her best, a win more option. If you aren't already winning, she provides nothing and in end game team fights she's just outperformed dramatically.