Gatekeepers is a fun little anime of the retro persuasion. Set in 1969, our heroes are students who have the ability to open "Gates" that grant them great powers of an elemental nature. So one person controls the gate for air/wind and another flame, etc. etc. They are recruited by a super secret organization that just happens to be built under the local high school (they have to create fake dust storms when mobilizing so no one guesses there's a super secret lair under the gym.) The foe is called just "the invaders," and they are sort of creepy. They look like MIBs and can fuse into various weird giant forms.
As I said, this is great fun. All done with a light touch and a fun sense of design. Being set in 1969, makes for an interesting time to fight funky aliens. There's an enormous sense of optimism. We could use more of that.