Madara Uchiha was the leader of the Uchiha clan prior to and after the formation of Konohagakure. After allying with his rival, Hashirama Senju, he also helped to found Konohagakure. Madara is considered by many notable individuals to be one of the most gifted shinobi in history and the most powerful that has ever been produced within the Uchiha clan.
He appeared when revived by the Impure World Resurrection.

Mustain is the one that took care of Melrose for 6 months. He uses music to fight and it's on the level of grandmaster.

Tensa Zangetsu is the manifested spirit of Ichigo Kurosaki's Zanpakutō in Bankai form that is seen in Ichigo's inner world. He has long ragged black hair. He is dressed in a tattered, black hooded cloak that flares out into ragged ends. He is agressive, but portrays cool and calm collectiveness.
He appeared in the inner world when Ichigo was trying to learn Final Getsuga Tenshō.

Apparently an easygoing, reckless and somehow comical high-school student known only due to his incapability to arrive on time for classes, he is in fact the result of an experiment made years before by a powerful crime syndicate lord.
The man, dying of old age, tried to use medical knowledge and advanced technology to revive himself as a youth, and thus create a cloning facility where he experimented with cloning humans and transferring memories and experience from a mind to another, so as to be reborn as an ultimate being. His plans failed however, as he died before his project could be brought into fruition, and thus the numerous Shou clones where all scattered far and wide in Japan, each of them focusing on a single area of expertise (be it stunt-driving, surgery, military training, and so on), until two of them encountered each other by chance, thus prompting all clones to reunite and, thanks to the shared experience of them all, vow to redeem Japan and destroy what they perceived as evil, in a similar way of a terror group.

Mustain