Unknown Alien serves as the final boss of both Metal Slug Mobile 4 and Metal Slug Universal. Its origins are unknown. It also bears a resemblance to a fallen Mother Brain, but is doubtful to be the same. Its weakpoint is its eye on the upper part of it. If hit enough times it will die, and the alien's lair will be destroyed.
- The stage is similar to the cell stage from Gradius Gaiden.
- The boss is like Gofer in Gradius II and Mother Brain from Metroid series.
|Heroes||Marco Rossi | Tarma Roving | Eri Kasamoto | Fio Germi | Trevor Spacey | Nadia Cassel | Ralf Jones | Clark Still | Leona Heidern | Walter Ryan | Tyra Elson | Roberto Nicola | Nathalie Neo | Alisa Stewart | Gimlet | Red Eye | Tequilla|
|Support||Hyakutaro | Rumi Aikawa | Madoka Aikawa | Utan | Issenman Tarou | Eris|
|Villains||Donald Morden | Allen O'Neil | Abul Abbas | Rootmars | Amadeus | Evil Spirit Incarnate | Ptolemaios | Invader Queen | Allen Jr. | Oguma | Macba | Hilde Garn | Unknown Alien|
|Instructors||Sophia | Margaret | Lilly | Mary | Cynthia|
|NPC||POWs | Parker | Satiko Suzuki | Scott Amundsen Jr. | Miner | Genie of Lamp | Orca | President | Sailor | Rebel NCO|
|Cameos||KOF Team | Battle Cats|
|Unused||Achilles | Tabomba | Ptolemaios|
|MSA Newcomers||Scotia Amundsen | Pharaoh | Red Goblin | El Dorado | Professor | Dragunov | MS-Alice | Abigail | Vatn | Yoshino | Odette | Beatriz | Caroline | Cleopatra | Vita | Annette | Jin|