So X said to Y:
I did tell you yesterday!
As far as I know, the word did there is to emphasize my point or tell him that I'm so sure I've told him the story yesterday. What I want to ask is, what's the word we may use to emphasize our point like that in a sentence with no verb? Like
You ARE the one who breaks it!
For emphasis, I capitalized the ARE, but is there any word that functions like did does in the previous sentence?