Reaper Bird, known in battle as "," (a single comma symbol) before its spare conditions are met, is an Amalgamate found in the True Lab, where it is disguised as an exclamation bubble that appears after gazing into a mirror. It is composed of parts from Astigmatism, Final Froggit, and Whimsalot. Its body is so long that about a third of it covers the dialogue box, which it retracts when attacking.
Reaper Bird's head is composed of the body of Astigmatism, with Astigmatism's legs serving as Reaper Bird's "beak". The small white areas at the top and bottom of its eye (which would typically be the sclera of a real eye) are repurposed as gnashing teeth which occasionally close over the pupil.
Reaper Bird's wings are made from Final Froggit's legs while its own legs are unnaturally-extended versions of Whimsalot's legs.
Everyman, Reaper Bird's main attack, appears to have the body of a cartoonish depiction of an anthropomorphic bird embryo. His head has a short appendage resembling a half-formed embryonic beak, and small black circles for eyes.
- Everyman appears to the right of the screen and has its head attacked by a swarm of butterflies, causing it to writhe in pain. The only way to be harmed by this attack is to run intentionally into the Everyman or the butterflies. (This is the first attack Reaper Bird uses and does not occur multiple times).
- The Everyman, now headless, shakily walks to the left of the screen as the butterflies fly off.
- The Everyman regenerates heads and tosses them, which home in towards the protagonist's SOUL.
The Reaper Bird can be spared by picking on it, praying, and mystifying it in any order.
- Its quotes, including its Check description, all appear to be garbled, meaningless mashed-up text at the beginning of the battle but are soon revealed to be quotes from Astigmatism, Final Froggit, and Whimsalot written over the top of each other.
- , [Neutral]
- Smells like a , [Neutral]
- You hum a familiar tune. But no one heard you. [Hum]
- You wash your hands. Nothing happened. [Clean]
- Your hands are clean enough to eat! [Cleaning again]
- You kneel and pray for safety. , remembers its conscience. [Pray]
- But , already remembered its conscience. [Praying again]
- You did something mysterious. , recognizes it has more to learn from this world. [Mystify]
- But , was already mystified. [Mystifying again]
- You picked on , . It seemed effective. [Pick on]
- But , was already picked on. [Picking on again]
- You kneel and pray for safety. Reaper Bird seems to remember something. [Pray after meeting spare conditions]
- You did something mysterious. Reaper Bird seems to remember something. [Mystify after meeting spare conditions]
- You pick on Reaper Bird. Reaper Bird seems to remember something. [Pick on after meeting spare conditions]
- Reaper Bird seems placated. [Mystify, Pray, and Pick on]
- This relentless future finally looks brighter and brighter. [Check]
- It is possible to encounter Whimsalot, Astigmatism and Final Froggit together in battle within the CORE. The flavor text will read "What a nightmare!", possibly alluding to the Reaper Bird itself.
- The name of the figure in Reaper Bird's attacks, Everyman, was confirmed in a deleted tweet by Toby Fox.
- Reaper Bird, along with the Memoryheads and the unknown figure(s), does not show up in the epilogue with the other Amalgamates.
- Reaper Bird's overworld sprite somewhat resembles the Chaos Buster attack used by Asriel Dreemurr in the True Pacifist Route's final boss battle.
- Reaper Bird's Check description is made up of parts of the Check descriptions of Final Froggit, Whimsalot, and Astigmatism.