Baldur's Gate 3's Patch 7 deputized a well-liked side character, the Tiefling bard Alfira, to full companion status. The only problem is that it's only temporary, and in the most «cursed monkey's paw» way imaginable.
Cenalexis_01 on the Baldur's Gate 3 subreddit figured out how to make it permanent, though, and the exploit requires the sort of obscene, beautiful abuse of Baldur's Gate 3's open-ended systems I've come to relish seeing. Spoilers ahead for the Dark Urge origin. One of the defining scenes of BG3's Dark Urge (Durge to their friends) origin is their unwilling murder of Alfira.
Partway through Act 1, she'll show up to your camp during a long rest and the Durge will unavoidably kill her—though it is possible, through shenanigans, to make stand-in Quil Grootslang show up and take the fall instead.
Patch 7 made it so that Alfira even joins your party and can be leveled up for a brief window, sparking some hope she was a surprise new companion added in the patch, but no dice: It was just meant to twist the knife a bit more.
But as cenalexis_01 has demonstrated, you can have Alfira as a full companion if you really want, it just entails twisting Baldur's Gate 3's game logic into a succulent Bavarian pretzel.