Tybalt

A mysterious warrior who aids the Capulet retainers, he tells them about the planned assassination of Benvolio's family and later aids them on his own ry?ba mounted in war gear. It is later revealed that he has a network of paid spies and informants throughout Neo Verona and that he has no qualms about killing. Tybalt is also one of the few outside the circle of Capulet retainers who knows both Juliet's secret as the last of the Capulets and her persona as the Red Whirlwind. Conrad may know the true scope of Tybalt's identity, but is still unsure. Tybalt's mother was tricked by Romeo's father, who abandoned her when he got her pregnant and married Romeo's mother.

That means Tybalt is Montague illegitimate son/Romeo's half brother. For his mother's sake, he has sworn to hate and avenge her by killing the Grand Duke of the Montague family.