He is the head of House Montague who overthrew the House of Capulet fourteen years prior to the start of the anime in order to become Prince of Neo Verona.
He is the father of Tybalt and Romeo, and the former husband of Lady Portia. He, an illegitimate Capulet offspring, and his mother (a prostitute) was born and raised into poverty. The death of his mother sparks his unrelenting and ferocious grudge against the Capulet name and his ambitions for being Duke. He acts coldly to all around him, only seeming to care about the Great Tree Escalus. His main goal in the series is to find Lord Capulet's surviving child and kill her before the people still loyal to the Capulets can rebel. He is fatally wounded late in the series by a mad Mercutio, moments before surrendering to Juliet. His given name, Leontes, is also shared by the main character of Shakespeare's The Winter's Tale.
In the play, Lord Montague is only a minor character appearing in only a few scenes of the play. While present, he is shown as being a caring and overprotective father to his son, showing no interest in destroying the Capulets but desiring for them to leave the Montagues alone instead. In the series, he is shown as one of the main antagonists and acts as Duke of Neo Verona instead of just being the head of his own household. He acts as an uncaring, ruthless, and power-hungry father and leader, as well as an illegitimate relation of the Capulet family.