There are 4 characters in the S-Tier: Ereshan, Nanna, Viola and Zero.

Ereshan
Ereshan is a 5* Burst DPS with a Darkness element. She deals decent single target damage but also deals quite a bit of AoE Charged damage, applying Phantom Mark’s to targets, increasing her damage dealt to them. She also possesses a unique traversal skill that allows her to teleport across gaps in terrain.
Her ultimate charges several times towards enemies, then pulls up to 4 enemies towards you dealing massive damage.

Nanna
Nanna is a 5* AoE Breaker with a Darkness element. She has a lot of varied attacks dependent on whether she’s in her Full Moon or New Moon state, whether she has Full Moon Blessing, she deals a lot of AoE damage the longer the fight goes on.
She’s honestly one of the most unique characters due to how ranged she plays – constantly throwing her blades forward to activate a variety of different effects.
Her ultimate throws a blade that creates a spatial distortion centered around it, causing large damage to all enemies pierced by the blades.

Viola
Viola is a 5* AoE Breaker with a Thunder element. While attacking she can unleash Original Tonal Range and Tonicized Tonal Range, which – while they exist on the battlefield, spread AoE damage every second continuously.
Her ultimate unleashes 2 electric nets that ensnare enemies, dealing massive damage to every enemy it touches.

Zero
Zero is a 4* Buffer/Supporter with a Light element. She deals quite a bit of damage, in spite of her focusing on the support role, with the ability to increase her entire teams’ attack for 20 seconds.
Her ultimate charges upward and deals a ton of damage to all enemies within the surrounding area upon her descent.

There are 6 characters in the A-Tier: Asti, Florence, Li Chi, Logos, Ming, and Qing.

Asti
Asti is a 4* Healer/Supporter with a Water element. She throws her umbrella, dealing AoE damage to every enemy in its path, generating Rainwave points, which enhance her abilities further. Her heals heal a total based on Asti’s max HP, and generate a Rainfall Zone that lasts for 20 seconds, healing allies every 2 seconds.
Her ultimate releases a tornado forward dealing large damage to everything it touches.

Florence
Florence is a 5* AoE/DoT/DPS with a Fire element. She deals massive AoE Derived damage 3 times to enemies in a large area after spinning like a beyblade and slashing downward. She also throws her greatsword forward, dealing AoE damage and knocking all enemies back. She can lunge forward and penetrate targets after throwing her sword.
Her ultimate swings her sword, delivering three enormous area slashes.

Li Chi
Li Chi is a 5* Single Target DPS with a Thunder element. He utilizes a Berserker gameplay style – he generates Battle Will as he fights, but gradually loses HP over the course of the battle, until he reaches 55%, after which he ceases to drain his own HP.
After generating 100 Battle Will points, he activates super saiyan, gaining a boost to his movement speed and dealing AoE Charged damage. There’s honestly a lot of complexity to his character – he’s probably one of the most difficult to understand and master.
His ultimate summons afterimages that attack enemies, with every hit from an afterimage restoring 10% of his HP. He’s honestly kinda insane.

Logos
Logos is a 5* DoT Healer, with a Light element. She – along with Nana, are my favorite Waifu’s in-game. Not only are they both powerful, highly unique, but they’re both absolutely stunning. She’s a very interesting character: She can fight, but can also summon Notes that follow and attack alongside allies, dealing Chase damage to enemies and healing allies every 0.5 seconds.
Her ultimate isn’t anything fancy, it just deals a ton of damage while flying all across the area.

Ming
Ming is a 4* AoE Breaker with a Fire element. While attacking she generates Candle Shadow’s, which after consumed summons flames around her for 10 seconds, dealing Chase damage to all enemies every 0.5 seconds.
Interestingly, she also links enemies together by Soul Thread, with both enemies sharing damage among eachother. Her ultimate summons a Lamp Ghost that deals damage to enemies in front of it.

Qing
Qing is a 5* AoE DPS with a Water element. She’s a fast attacker, hitting an enemy in her path, dealing Enhanced damage. Activating Abyssal Calm allows her to perform a circular AoE on the spot, dealing Enhanced damage.
Hitting enemies transforms her abilities into powerful AoE Enhanced damage that strikes all enemies in a straight line.
Her ultimate leaps into the air and slashes downward, releasing a wave of water.

There are 5 characters in the B-Tier: Azi, Enki, Hephae, Ninsar and Shamash.

Azi
Azi is a 4* Support/Debuffer with a Darkness element. Azi’s relatively unique in so that she fires a rocket launcher that marks enemies with Arrow Wound, causing enemies to take double damage.
She also deploys drones to bombard enemies in a designated area, dealing Enhanced damage, resulting in a large explosion.
Her ultimate causes an orbital cannon to strike down every enemy in the immediate area.

Enki
Enki is a 4* Debuffer/Breaker with a Water element. He’s a very basic character – he deals Regular attack damage with both his abilities, sending Energy Orbs flying forward that have a chance of exploding, dealing AoE damage.
His ultimate throws an expanded Energy Orb, after which he leaps into the air, causing it to explode and deal additional damage.

Hephae
Hephae is a 4* Anomaly/Support/Shielder with a Fire element. She jumps around a LOT and deals damage depending on where she lands. She sets up a Forging Zone which when triggered, deals a small amount of damage, but increases all party members’ damage resistance for 20 seconds, stacking up to 5 times.
Her ultimate swings her hammer around, followed by a charged strike on a target.

Ninsar
Ninsar is a 4* Support/Shielder with a Darkness element. She’s a little more unique than most characters, utilizing her Golem to attack. She has a focus on AoE damage – both her and her Golem. Upon casting her 2nd skill she activates a 15 second barrier that grants allies a shield based on her max HP that refreshes every 2 seconds.
Her ultimate commands her Golem to unleash a Charged strike that deals massive damage in a wide area.

Shamash
Shamash is a 4* DPS/Tank with a Light element. He can block attacks with his stone tablet, after generating enough rage, and a successful block, you deal a large chunk of Enhanced damage. He also throws his weapon ahead of him, running through enemies, dealing damage to the targets hit.
His ultimate summons a radiant ring in front of him, causing a massive explosion.

There are 2 characters in the C-Tier: Xin and Wushi.

Wushi
Wushi is a 4* Single Target DPS with a Darkness element. He’s a little ninja, utilizing teleports to get above a single enemy and afflict them with Dark Scar. He can enhance his own attack in various ways, and unleash AoE Chase Damage via his Dark Blade effect.
His ultimate releases 8 flying blades that pierce through enemies, dealing damage to everything hit.

Xin
Xin is a 4* Burst DPS with a Thunder element. She throws her spear to attack, piercing enemies in her path. She gains Lightning Dash through successful hits on targets afflicted by Galvanic Electricity, which allows her to unleash a flurry of stabs, dealing Derived damage.
Her ultimate is relatively basic, jumping into the air to perform a wide-range stab, followed by a cleaving strike.

And that’s every character, sorted by utility and importance. Hopefully it helps set you off on the correct path to dominance through time.