If you're referring to the scene after Romeo is banished, and he's with Friar Lawrence in his cell, then it would be Friar Lawrence who called Romeo a coward. However, Romeo is called a coward more than once in the book if I recall correctly, so that may not be the time you were asking about.
Mercutio calls Tybalt a coward in Romeo and Juliet.
mercutio accuses romeo of being a coward because romeo had refused to fight Tybalt and to mercutio that made him a coward all though romeo had a reason not to fight because Tybalt was family to him then but no one had known it beside romeo Juliet the nurse and friar Laurence
He thinks Romeo is a coward.
mercutio
Benvolio
Mercutio calls Tybalt a coward in Romeo and Juliet.
He thinks Romeo is a coward.
mercutio accuses romeo of being a coward because romeo had refused to fight Tybalt and to mercutio that made him a coward all though romeo had a reason not to fight because Tybalt was family to him then but no one had known it beside romeo Juliet the nurse and friar Laurence
He thinks Romeo is a coward.
Romeo, oh Romeo! Where arth thou Romeo?
In the passage in Romeo and Juliet, Tybalt insults Romeo by calling him a "villain," disrespecting him because he is a Montague. He further provokes Romeo by referring to his rapier, a type of sword, implying that Romeo is a coward for not defending his honor.
courage courage Its a coward or if there being racist... Chinese
slay him
mercutio
He thinks he's a coward. "Oh, calm, dishonourable, vile submission."
He calls him a villain.
He calls her the "son"