Chikyuu Boueigun, or Earth Defense Force, is a series of low-budget horde shooters developed by Sandlot and published by D3 Publisher. In each game, the earth has been invaded by aliens, who deploy such B-movie tropes as giant insects, flying saucers, huge combat robots, giant green men, and godzillas against humanity. In most games, you play a normal infantryman. In 4.1 and 5, you can choose between normal infantryman, a slow-but-strong power-armour soldier, a fast-but-fragile jetpack girl, or the guy who calls in airstrikes. In any case, you acquire new and stronger weapons in a sort of "gacha" system; slain enemies occasionally drop crates that contain one random weapon, and you won't find out what it is until the mission ends. There's a lot of weapon variety, and you choose 2 weapons to bring with you into each mission, so there's some good fun to be had exploring for synergistic loadouts.

The story is absurd, hammy B-movie fare, written and dubbed with a lot of heart. Most of the games are 4-player coop. It's one of my favourite serieses of all time. I'd recommend starting at 5, because it's available on Steam, it has the most QoL (it's one of those series where each new game feels like the previous game, just.. more and better) and they reboot the chronology every odd game anyway so you're not missing out on anything much. And also because it has aliens who look exactly like humans! 🐸