Unfortunately this is one of those 'Larian simply forgot' answers.

Us survived no doubt as it wasn't tethered to the Absolute and the lobotomy (if it's given) at the beginning of the game ensures it never again being able to join any hive-mind. But it was a very minor character to be carried around as a familiar spell item with the only voice-line post-rescue being its kitty disguise explanation, basically to be a very tiny cameo.

Could send feedback to remind them. They made some extra additions to the endgame by including other minor factions and characters with Update #7, so could ensure Us is present as well. But if not; personally I like to think it went on to live with the Emperor since he isn't present either. Like the mindflayer Nihilor who had intellect devourer pets;

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