Brutus and Casca were both part of a conspiracy to kill Julius Caesar.
Brutus was called "the younger" and Casca was called "envious Casca".
Brutus reputation and relationship to the conspirators is clear on lines 162- 165. On these lines, Casca says that if Brutus supports the conspiracy, people will see why it's important and why it is not something offensive. Furthermore, Brutus is mentioned as someone who "sits high in all the people's hearts".
Finally, the conspirators need Brutus because of his great reputation and how it would give the plan some credibility.
