Alien Enemies (エイリアン eirian) are a type of Enemy Unit. They are introduced in Chapter 1 of Into The Future. They appear in Into the Future, Cats of the Cosmos, Stories of Legend and some events. They are rather strong, generally don't give much money, and have a peculiarity: Their Health and Damage are multiplied by 700%, making them EXTREMELY strong, unless a player has all Superior anti-Alien treasures. Each 100% Treasure reduces this multiplier by 100% (66% reduces it by 66%, 33% by 33%, and so on).
Alien enemies introduced in Cats of the Cosmos are "Starred", which means they have a 1600% strength magnification by default and are unaffected by Into the Future's anti-Alien Treasures. However, Cats of the Cosmos has many Treasures that reduce the power of Starred Aliens. Like the Into the Future Treasures, each 100% Treasure reduces their power by 100%. The stared aliens also have a special ability most commonly having a chance to teleport cats. Not only that, but half of them also gain a barrier that protects them from damage. If the damage the starred alien takes is less than the barrier's health, the damage is completely nullified. The only way to break it is with a high damage attack like A. Bahamut or a cat with barrier breaker.