Who is Messala in Julius Caesar and why is he an important character?

In "Julius Caesar," Messala is a friend to Brutus and Cassius and an officer in Brutus's army who brings Brutus news from Rome, confirming for Brutus his letters that

young Octavius and Mark Antony/Comd down upon us with a mighty power,/Bending their expedition toward Philippi (IV,iii,167-169)

Also, Messala informs Brutus that the triumvirate have put to death a hundred senators, with Cicero as one of the political victims.  When Messala asks Brutus if he has received any letters from his wife, Brutus replies that he has not.  It is left to Messala to inform Brutus that, tragically, his beloved Portia "is dead, and by strange manner" (IV,iii,188).  Caring the burdens of her husband's actions, Portia cannot bear up and commits suicide.  While Brutus does not at first react to this news except to say

Why, farewell, Portia.  We must die, Messala./With meditating that she must die once,/I have the patience to endure it now. (IV,iii,,189-191)

his words seem to echo those of Caesar when Calpurnia begs him to not go to the Senate: "The valiant never taste of death but once.(II,ii,33) and stir his guilt as he, shortly thereafter, sees the ghost of Caesar.  Then, in Act V, at the battle in Philippi as informed by Messala, Brutus himself commits suicide.