At the very start of Baldur's Gate 3, you can recruit an ally who will be available to help you out throughout your journey, but he's extremely easy to miss. He also vanishes after the very start of the game and can be found later in the game, though he's easily missed. We've got all the details on where you can find the Intellect Devourer, and how to keep him on your side.
Freeing The Intellect Devourer
In order to free the Intellect Devourer, you have to do what will likely be your very first Skill Check. You've offered multiple different skills to use, so as long as you play towards the strengths of your chosen class and race you should be fine. Make sure you don't mutilate the brain when you're given the option, and the Intellect Devourer (who calls himself Us) will join you in your fight. Make sure you DON'T attack any other Intellect Devourers on the Nautiloid Ship in the prologue though, as Us will identify you as a threat if you do.
Once the Nautiloid Ship crashes, Us will disappear. You can find him again inside the Mind Flayer Colony towards the end of Act 2, and you can free him. Once you've freed him he'll join you full-time, and you'll be able to summon him in battle similar to how you can summon Scratch and the Owlbear.