Which character has a little time on screen but is very important to the plot ?

I watched "John Wick" for the first time few hours ago (I'm not a trending guy) and it was very good. I realized that all the movie exist because of the death of a dog we see for barely 5 cumulatives minutes. I don't know if there's a noun for those kind of character important to the plot but with a little time on screen and I fail to find other examples of that. So I ask here. via /r/movies https://ift.tt/2UZAfTr