As you traverse the Mindflayer's ship at the start of Baldur's Gate 3, you'll come across a few companions, namely Us the Brain and Lae'zel the Githyanki. As you near the end of the act, you'll come across a cleric named Shadowheart who's trapped in one of the Mindflayer's pods. She can be recruited into your party if you find a way to free her, so here's how to rescue Shadowheart from the pod.
How to free Shadowheart
Towards the end of the first act upon the Nautiloid ship, you'll enter a room on the way to the helm with a restoration point to the right and a glowing red pod to the left. Interact with the pod to speak to Shadowheart, who'll beg you to let her out. There's a console to the right of her that you can interact with, and you'll discover that there's an empty socket and that the rune that fills it is missing.
Head to the room to the left of the pod and console. Inside, there's a pod, a console, and a corpse—just what you'd expect, really. Interacting with the console turns the person in the pod into a newborn Mindflayer, so that's nice. If you search the body at the back of the room, you'll find an Eldritch Rune.
Once you've picked up the rune, interact with the console again to inset free Shadowheart and she will join your party, as long as you pass an Illithid/Wisdom check. It's only a two, so unless you're super unlucky, you'll pass the check but maybe save beforehand just in case.
She'll stay with you for the remainder of this section and you can find her again once you reach the Ravaged Beach. She'll acknowledge the fact you rescued her and thank you for it, and you'll gain some approval.