Balthasar and Abram are servants of the Montague family. After the initial fight in the town square, we don't really hear from Abram again. Balthasar, however, is Romeo's valet, and is featured throughout the play. When Romeo kills Tybalt in Act 3, he is standing there in shock, and it is Balthasar that drags him off to the safety of Friar Laurence's cell. Balthasar always has Romeo's back. He's like Romeo's guardian angel. Balthasar is the one who sees Juliet's funeral and tells Romeo that Juliet is dead. He tries to get Romeo to slow down, not to rush to Verona, to stop acting crazy. Romeo doesn't listen. Balthasar, even helps Romeo get back into Verona, takes a letter meant for Romeo's father, and leaves when Romeo asks him too, even though he knows Romeo is going to kill himself. Throughout the whole play, Balthasar tries to take care of Romeo.
NOTE: In the modern version of the film, Abram is cast as a Capulet. It is not unusual for directors to take such liberties with the minor characters.