As you travel through the Forgotten Realms, you'll discover a lot of strange things. There are weird creatures, enchanted items, and big swirly scary things in Baldur's Gate 3. You can actually find one of your potential companions in one of these big swirly scary things, and it's up to you to get him out so that you can add a new member to your party.
Gale the Wizard is inside a fast travel point, having seemingly gotten stuck there during an attempted teleportation spell. His arm is sticking out of the runestone, and you can perform a strength check to pull him out. If you do this, the Wizard will be so elated at your saving him that he'll ask to join your party, and you'll have a wizard to back you up.
As a wizard, Gale is able to use powerful area-of-effect spells in case he is surrounded by enemies, and can easily blast away enemies with powerful enchantments. However, he's fairly useless when it comes to close-range combat, so keep him at a distance from any enemies or they will very quickly whittle down his health bar and put him on death-saving throws within pretty much one or two hits.