Count Melun is a pretty minor character. He's one of the French lords who join Louis on his invasion of England in Act V of the play.
Despite his small role, however, Melun actually plays a pivotal role in the play. When he is mortally wounded in the battle between King John's forces and those of Louis and the rebel English lords, Melun approaches Salisbury, Pembroke, and Lord Bigot (the English rebels) and tells them that Louis plans to murder them if the expedition is successful.
Melun says that he is revealing this for two reasons: (1) because he is about to die and will have a better chance to getting to heaven if he tells the truth, and (2) because his grandfather was English, so he has some sympathy for the English cause.
Melun's act has major consequences because it makes the English lords abandon Louis and swear allegiance again to John. This is probably the main reason why Louis has to abandon his invasion, thus putting England in a safe position at the end of the play.