I want to create a game with a multiple character storyline (like in Sonic Adventure). Is there a way to script this?
It's not a matter of whether or not Unity can do it. It's a matter of whether or not you can. There's a lot of foundational work you'll need to do, and you'll need scripting know-how. There isn't an out of the box solution.
Unity can do basically anything you want it to... if you are skilled enough. Its YOU who has to come up with the multiple storylines. Its YOU who has to do the scene building. Its YOU who has to script everything. Its YOU who has to do the millions of things that are involved in creating a game. Then and only then once all those things are complete, after all those months of your life are spent, will unity in a couple seconds do a bit of computer magic that will turn a ton of words and thousands of polygons into a full functioning game. Its really not about what Unity can do, its about what you can do.
A simple method is to add two static variables. Over the course of the the game have these two variables add up based on decisions, items, etc. Once you reach a certain point, calculate which is higher and based off that, take the player to a different scene. it's about the same as setting up a Game over function.