Spoiler alert for a section of the storyline: While this has as few spoilers as possible, there are a few.While creating your character in Baldur's Gate 3, you will notice that after creating your character, you will be asked to design another character in the character creation menu. The game will call this character your "Guardian," but will not tell you anything else bout the being. You will be able to pick their Race and what they look like; however, you cannot pick their Background or Class. You may also hear this character referred to as your "Dream Visitor."
Baldur’s Gate 3: What Is The Guardian
In Baldur's Gate 3's Character Creation, you will be asked to create your Guardian. Here is a guide explaining that Guardian.