Compiler - Manga
Summary
COMPILER is a story about the Igarashi brothers Nachi and Toshi, and two beauties from another dimension, Compiler and Assembler. Originally sent to conquer Earth through a game of destruction and creation, Compiler and Assembler ended up deciding that living in our world is more fun than conquering it. They moved in with Nachi and Toshi. A growing relationship began to develop between Nachi and Compiler, and Toshi and Assembler, though none of them can bring themselves to admit it outright.
Meanwhile, the people who sent Compiler and Assembler to conquer Earth were outraged by their apparent betrayal. They sent two assassins, Bios and Directory, to deal with the renegades and proceed with the invasion plan. And so, the misadventures begin...
... and the second season of the Anime did a better job of pacing than I thought they would. I think they moved the Elf village arc from before hostilities with the Principality began to the inter-war period, aka the start of the second season, and used that to pad it out. Good call if that's what they did. In any event. Good season.