Lucius is a young boy who is a servant to Brutus. He may be a slave-boy. His main function in the play seems to be to give Brutus, and later Portia, someone to talk to in scenes where otherwise these characters would be all alone and would either have to remain silent or indulge in more soliloquies. If a character is all alone on a stage, the audience has no way of knowing what he or she is thinking or feeling (unless they tells us in a soliloquy). Although Lucius is obviously very young, he seems to hold an important position in Brutus's household. He is treated like a favorite. Both Brutus and Portia rely on him and trust him. In Act 2, Scene 4, it is Lucius, a mere boy, whom Portia sends to the Senate House to see what her husband is doing and report back to her. This scene is engaging because it is both crucial and comical. Portia wants Lucius to run post-haste to the Senate House but can't tell him why she is sending him there.
I prithee, boy, run to the Senate House,
Stay not to answer me, but get thee gone!
Why dost thou stay?
To know my errand, madam.
I would have had thee there and here again
Ere I can tell thee what thou shouldst do there.
Portia is by now fully informed about the assassination plot against Julius Caesar. Brutus succumbs to her pleadings in Act 2. Scene 1, and tells her:
Hark, hark, one knocks. Portia, go in awhile,
And by and by thy bosom shall partake
The secrets of my heart.
All my engagements I will construe to thee,
All the charactery of my sad brows.
Leave me with haste.
The playwright does not show Brutus doing as he promises because that would be redundant. The audience already knows all the secrets of Brutus's heart. Portia has obviously been fully informed as her husband promised. However, she must keep strict secrecy and cannot tell Lucius that she wants him to hurry to the Senate House because she wants to learn whether her husband's plot against Caesar is successful or thwarted.
Brutus has to flee from Rome because of Antony's inflammatory funeral speech. Brutus takes Lucius with him, and Lucius appears again in Act 4, Scene 2 in Brutus's tent in his army's camp near Sardis. Lucius acts as Brutus's personal servant and keeps him company after Cassius and all the others are gone. At Brutus's request, Lucius plays a stringed instrument and sings for him.
Lucius plays a tune and sings a song. He slowly falls asleep, and as he does so, the music stops.
Here again, Lucius gives Brutus someone to talk to. When Lucius falls asleep, Brutus is effectively alone again, but then the Ghost of Caesar enters almost immediately.