Brutus, as a character, is in many respects divided. In contrast to Cassius, who loathes Caesar, Brutus and Caesar are depicted as genuine friends with one another. And yet Brutus becomes convinced that Caesar represents a threat to the Roman Republic, and on these grounds he joins the conspirators in their plot. He has no personal vendetta or quarrel with Caesar himself, but for Brutus, his personal relationship with Caesar is placed in subservience to what he views as his patriotic duty and loyalty to the State, and he acts accordingly.
At the same time, while Brutus's actions and relationships do tend to be genuine, he is contrasted with the more manipulative personality of Cassius, who aims to draw Brutus into the plot. Cassius is shown actively deceiving Brutus, forging various letters and sending them to Brutus, all in order to convince him that Caesar is a threat that must be dealt with. In this respect, even in his betrayal and murder of Caesar, Brutus seems to hold a higher regard for Caesar than Cassius extends towards himself, given the way Cassius is willing to manipulate Brutus in this manner. That Brutus then sided with Cassius and his allies over Caesar is a noticeable nuance in the play.
Thus, in Brutus's relationship with Caesar, you can observe friendship and good will clashing with political idealism and patriotism, and in his relationship with Cassius, you can observe a kind of two-faced relationship, where Cassius is not above taking advantage of Brutus, manipulating him to advance his own designs.