Brutus was first and foremost an honorable man, putting the safety of Rome above everything else. His three most noticeable characteristics were his honor, his naivete, and his stoicism. However, his honor honesty, and trustfulness eventually became the things that killed him.
First of all, Brutus is a stoic. He and his wife Portia are both very stoic, and they don't show emotions towards things. The most striking instance of Brutus' stoicism is when Portia commits suicide. Cassius is having a hard time accepting Portia's death, but Brutus just shrugs it off. He immediately says to Cassius "Speak no more of her. Give me a bowl of wine" (V. iii. 157-158). Another example is when Portia stabs herself in the thigh. Even after this Brutus does not tell her what his plan is and instead immediately sends her off. Finally, when facing the possibility of taking his own life, he shows little emotion and instead still thinks out everything extremely carefully.
Brutus' sense of honor and his love for Rome was his most striking quality. With those qualities also comes a sense of honesty. The best example of Brutus' honor was his decision to join the conspirators. Although he was a good friend to Caesar and Caesar had not done anything bad yet, the very threat of Caesar becoming a tyrant led Brutus to joining the conspirators. Cassius said Brutus could be swayed with his honor, meaning Brutus values his honor so much that he places it above all else (I. ii. 304-308). Besides from joining the conspiracy, Brutus also himself said he would look at all public matters equally and that "I (Brutus) love the name of honor more than I fear death" (I. ii. 85-89). Upon Brutus' death, Marc Antony called Brutus the "noblest Roman of them all" (V. v. 68-75). He said all the conspirators except Brutus killed Caesar out of envy. Even when committing a crime so severe as murder, Brutus suggested to the conspirators that they should "carve him (Caesar) as a dish