Since story-driven games tend to be several hours long, it can be hard to pay attention to everything that is happening. Your focus is almost exclusively dedicated to the main cast of characters, and all the little details happening in the background can easily slip past your radar.
However, just because you don't see what is going on, it doesn't mean it is not important. Some of the key elements of the plotline can play out in the background and catch you by surprise at a later point in the story. The best part is that sometimes it's not even a major character that makes these types of moments happen. Even an episodic companion has the capability of changing the course of the entire game.
The change can also be so severe that it bleeds into the game's sequel, or it can downright make the sequel happen! These characters have defined the future of their titles without you realizing it, whether through notes and context clues left by the game's developers or by a single moment that did more to the story than you might guess at first glance.
10. Garl Death-Hand - Fallout
Those who have played both Fallout 1 and 2 realize how important Tandi is to the Fallout universe as a whole. Along with her father, she is the founder of the New California Republic and one of its first elected presidents. The faction plays an important role in all Fallout games taking place on the west coast of the US and gets mentioned in almost every other title.
However, as much as Tandi's involvement in the creation of the NCR is obvious, her decision to unite the people of the wasteland under its banner wouldn't have been made if it weren't for the choices made by the ruthless raider who kidnapped her, Garl Death-Hand.
Yes, ironically, it is Tandi's enemy who motivates her to make the change. Garl's gang was the chief reason why the settlements of Fallout 1 couldn't unite. When Garl decides to kidnap Tandi, he effectively brings the wrath of the Vault Dweller upon his people and causes the destruction of the major threat looming over Sandy Shores.
Additionally, kidnapping Tandi probably opened her eyes to the dangers awaiting her settlement and motivated her to stay with them instead of running away on an adventure like she always wanted.