Most players hit a wall in Limbo 6 or Mane's Bulletin, wipe to a raid-wide nuke, and immediately blame their DPS. They are wrong. You are leaking Action Points (AP) because you fundamentally misunderstand how sustain scaling works in Reverse: 1999.
To the 7,000 of you subscribed to the RewardPact YouTube channel, you already know we do not rely on "feelings" to build teams. We use data. Right now, the community is split between two top-tier sustains: Tooth Fairy (the 6-Star Star Afflatus) and Yenisei (the 5-Star Water Afflatus). Tier lists will blindly tell you to slot Tooth Fairy everywhere. But if you actually run the numbers on Critical Healing Expected Value (EV) versus Effective HP (eHP) generation, Yenisei mathematically outperforms her in specific, high-stress environments.
Let's break down the exact math, the Resonance blocks, and the AP economy behind the game's two best healers.
The Core Breakdown: ATK vs. Max HP Scaling
The primary difference between these two Arcanists lies in their core healing calculation. Healing in Reverse: 1999 is heavily modified by the base stat the incantation calls for, the user's Critical Rate, and the Psychube modifiers.
Tooth Fairy: The Critical ATK Engine
Tooth Fairy scales her healing off her ATK stat. Her core healing incantation, Lullaby, heals all allies for 80% of her ATK at a 1-star cast.
Let's look at a max-invested Tooth Fairy (Insight 3, Level 60, Resonance 10):
Base ATK: ~1150
Psychube: Beyond Wonderland (Level 60) provides +370 ATK and +16% Healing Done.
Total ATK: 1520
Base Heal Calculation: 1520 * 0.80 = 1216 base healing.
Healing Done Modifier: 1216 * 1.16 = 1410 actual base heal per target.
However, Tooth Fairy is designed to crit. With her custom Resonance 10 build prioritizing Crit Rate blocks, plus Beyond Wonderland, she comfortably sits at a ~60% Crit Rate. Her Critical Damage multiplier hovers around 145%.
The Expected Value (EV) of Lullaby: When a heal crits, it multiplies the base heal by the Crit DMG modifier.
Critical Heal: 1410 * 1.45 = 2044
Factoring in the 60% consistency, the EV per cast is roughly 1790 HP per ally.
Yenisei: The Max HP Behemoth
Yenisei ignores ATK entirely. Her healing incantation, Words of Water, heals all allies for 8% of her Max HP at a 1-star cast.
Let's map out a max-invested Yenisei (Insight 3, Level 60, Resonance 10):
Base HP: ~7200
Psychube: That Inquisitive Dear (Level 60) provides +2200 HP and +18% Healing Done.
Resonance 10 (HP Focus): Adds roughly +2400 HP.
Total HP: 11800
Base Heal Calculation: 11800 * 0.08 = 944 base healing.
Healing Done Modifier: 944 * 1.18 = 1113 actual base heal per target.
At first glance, 1113 is significantly lower than Tooth Fairy's 1410. Furthermore, Yenisei does not naturally build Crit Rate in her Resonance grid because she requires HP and Damage Reduction blocks to survive. Her Crit EV is negligible.
If we stop the math here, Tooth Fairy wins. But raw healing is a trap.
The eHP Argument: Shields and Damage Mitigation
Healing only matters if your character survives the hit. This introduces the concept of Effective HP (eHP).
Yenisei's Damage Mitigation Mechanics
Yenisei completely shifts the math when you factor in the secondary effects of her kit.
Sturdiness: A 2-star cast of Words of Water grants 1 stack of Sturdiness (-25% Damage Taken for one attack) to the ally with the lowest HP percentage.
Water Flows Upward (Ultimate): Grants all allies a shield equal to 20% of Yenisei's Max HP for 2 rounds, plus 1 stack of Immunity.
With an 11800 Max HP Yenisei, her Ultimate generates a 2360 HP shield for the entire team.
The Mane's Bulletin Scenario: Imagine Turn 20 against the Marsh Creation boss. The boss queues up an Ultimate that deals 6000 raw damage to your main carry.
Your carry has 5500 Max HP.
With Tooth Fairy, your carry takes 6000 damage and dies. The 2044 critical heal on the next turn is useless because the target is dead.
With Yenisei, your carry has a 2360 HP shield and Sturdiness (-25% damage). The 6000 damage is mitigated to 4500. The shield absorbs 2360. Your carry takes 2140 actual damage. They survive with 3360 HP.
Math dictates that flat damage reduction and shielding scale infinitely better against high-end content than raw reactive healing.
The AP Economy and DPS Amplification
We cannot evaluate these units in a vacuum. We have to analyze what they cost your team in Action Points.
Tooth Fairy's 0-AP Value
This is where Tooth Fairy justifies her status as a top-tier 6-Star. Her core mechanic, Empty Gums, operates completely passively.
Every time an ally uses an incantation, she gains a stack of Baby Teeth.
At 5 stacks, she automatically inflicts Confusion (-15% Crit Resist) on all enemies before the round begins.
Cost: 0 AP.
This passive translates to a direct, massive increase in your main DPS's output. Characters like Jiu Niangzi or Melania spike heavily in damage when they are guaranteed to crit. Tooth Fairy acts as a sub-buffer without ever needing to play a card. You only cast Lullaby when you absolutely need the HP.
Yenisei's AP Tax
Yenisei is heavily reliant on her Ultimate to provide her eHP value. To get her Ultimate, she needs Moxie. To get Moxie, you must cast or merge her cards.
Every card you play for Yenisei is a card you are NOT playing for your main DPS.
This is an AP Tax. You are trading raw team damage output for guaranteed survival.
Practical Application
How do you apply these numbers to your daily grind?
Artificial Somnambulism (Limbo)
Winner: Tooth Fairy. Limbo is a DPS check. You have strict turn limits. Surviving with full HP on Turn 12 does not matter if you fail to kill the boss. Tooth Fairy's 0-AP Confusion debuff accelerates your kill times. You only need enough healing to barely survive the fight. Reactive critical healing is perfectly suited for Limbo's predictable damage patterns.
Mane's Bulletin (Raid Bosses)
Winner: Yenisei (In late turns). Mane's Bulletin scales infinitely. By Turn 20, bosses gain massive damage modifiers that will one-shot heavily invested 6-Star carries. You cannot out-heal a one-shot. You must mitigate it. Yenisei's Immunity blocks fatal debuffs, and her 20% Max HP Shield acts as an artificial health bar. If your goal is to push SSS rank, Yenisei provides the mathematical ceiling for survival.
UTTU Flash Gathering
Tie. UTTU depends entirely on the current season's FAME cards. If the cards provide massive ATK buffs, Tooth Fairy scales harder. If the cards provide Max HP or demand you survive burst mechanics, slot Yenisei.
The F2P Perspective: Investment Math
Let's talk account economy. Gacha games are resource management simulators.
As a 5-Star, Yenisei is significantly cheaper to take to Insight 3. More importantly, as a Free-to-Play player, you will naturally pull duplicates of her over time.
Portray 2 (P2): Extends her Ultimate shield duration to 3 rounds. This is a massive buff to your AP economy, meaning you have to cast her cards less often to maintain the shield.
Portray 5 (P5): Increases her base healing multipliers.
A P5 Yenisei is mathematically superior in pure sustain to a P0 Tooth Fairy. F2P players must stop sleeping on 5-Star max-portray units. You are doing your account a disservice by forcing a P0 6-Star into a raid environment where a P5 5-Star offers more eHP.
Conversely, pulling for Tooth Fairy is an investment in damage, not just healing. If your account lacks buffers like Shamane or An-an Lee, Tooth Fairy consolidates the roles of healer and debuffer into a single slot.
The Final Verdict
Stop treating healers as generic green number generators. Look at the boss mechanics, look at your team's base HP, and look at your AP economy.
If you need to end the fight in 10 turns and your carry scales on critical hits, use Tooth Fairy. Her passive AP efficiency is unmatched.
If you are fighting a boss that ramps damage infinitely and threatens to one-shot your team through raw stats, build Yenisei. Stack HP Resonance blocks, equip That Inquisitive Dear, and abuse her shielding mechanics.
Do not brick your account by following generic advice. Build for the math. If you want to see exactly how these two scale alongside the rest of the roster, check out the updated master tier lists over on the RewardPact channel. Keep optimizing.