Quintus and Martius (not to be confused with Mutius) are Titus's sons. They are framed for Bassianus's murder and executed.
Martius and Quintus are an obvious foil for some of the other pairs of brothers in the play. (See "Character Roles" for more on this.) When Martius falls into the pit in the middle of the forest, his self-sacrificing brother Quintus tries desperately to pull him out: "Thy hand once more; I will not loose again, / Till thou art here aloft or I below," he says. When it becomes clear that Quintus won't be able to rescue his brother, he tumbles in the pit after him: "Thou canst not come to me – I come to thee," he declares (2.3.7).
Compare this loving and loyal behavior to the way Chiron and Demetrius bicker over who should get to go after Lavinia or to the way Bassianus and Saturninus fight over who gets to be emperor.