Here is the full list of all 19 Undernauts: Labyrinth of Yomi achievements.
[Legendary Undernaut Mission] You escaped from Yomi.
[Legendary Undernaut Mission] You established yourself as a collector of S-rarity weapons and armor.
[Legendary Undernaut Mission] You revived the Divine Child.
[Expert Undernaut Mission] You exited Yomi and returned to the surface.
[Expert Undernaut Mission] You disposed of the Slothful Sinner.
[Expert Undernaut Mission] You established yourself as a collector of A-rarity weapons and armor.
[Expert Undernaut Mission] You disposed of the Wrathful Sinner.
[Expert Undernaut Mission] You disposed of the Prideful Sinner.
[Standard Undernaut Mission] You established yourself as a collector of B-rarity weapons and armor.
[Standard Undernaut Mission] You disposed of the Desirous Sinner.
[Standard Undernaut Mission] You disposed of the Anguished Sinner.
[Standard Undernaut Mission] You disposed of the Envious Sinner.
[Novice Undernaut Mission] You disposed of the Gluttonous Sinner.
[Novice Undernaut Mission] You established yourself as a collector of C-rarity weapons and armor.
[Novice Undernaut Mission] You disposed of the Reckless Sinner.
[Novice Undernaut Mission] You disposed of the Violent Sinner.
[Rookie Undernaut Mission] You established yourself as a collector of D-rarity weapons and armor.
[Rookie Undernaut Mission] You reached the other company's camp.
[Rookie Undernaut Mission] You returned to your company's camp.