In the Destiny 2 Final Shape Story, the Guardians are the central characters. They are joined by the likes of Ikora Rey, Zavala, and Cayde - 6 (if his story elements are still relevant). There are also new characters introduced that are either allies or enemies. For example, some alien species leaders that hold keys to understanding the new threat. The Exo Stranger is another character that might be important as she often has insights into the strange events occurring. Each of these characters brings something different to the story, whether it's combat expertise, knowledge of the lore, or strategic planning.
The main plot revolves around the Guardians facing a new and ultimate threat that could reshape the entire Destiny universe. They have to battle against powerful foes and make difficult choices to save the world as they know it.
The Destiny 2 story campaign length isn't fixed. If you rush through it, you might finish in 15 hours or so. But if you take your time, do side quests, and explore every nook and cranny, it could easily stretch to 40 hours or more.
You need to follow the quest markers and instructions. Pay attention to the dialogue and objectives given to you. It's pretty straightforward if you do that.
Main story missions in Destiny are often highlighted in the quest log or mission list with more prominent display and detailed descriptions. Also, they might be introduced by key NPCs with important dialogues.