Merlin was a wizard who lived during the medieval era. Little is known of his past, but he was a member of King Arthur's court, and arguably one of the most powerful wizards of all time.
Salazar Slytherin
Professor Salazar Slytherin was a pure-blood wizard of medieval times. He was a Parselmouth (a wizard with the rare ability to speak with snakes) and skilled at Legilimency. He was one of the four founders of Hogwarts School of Witchcraft and Wizardry and the former best friend of Godric Gryffindor, a fellow founder. Slytherin didn't trust Muggle-born students, and he expressed his opinion not to accept them at Hogwarts. However, when the other Hogwarts founders didn't agree, Slytherin created the Chamber of Secrets, concealing a basilisk inside. Afterwards he left the school, never to return. He died sometime in the Middle Ages.
Rowena Ravenclaw
Professor Rowena Ravenclaw (fl. c. 993 – 1000) was a Scottish witch, who lived in the early Middle Ages. Noted for her intelligence and creativity and regarded as one of the greatest witches of the age, Ravenclaw was one of the four founders of Hogwarts School of Witchcraft and Wizardry along with Godric Gryffindor, Helga Hufflepuff and Salazar Slytherin, as well as the namesake of the Ravenclaw House. Ravenclaw was "beautiful yet slightly intimidating."
Ravenclaw died sometime into the eleventh century, after she fell fatally ill. Legend had it that a broken heart is the cause — because her daughter, Helena, had run away with her diadem.
Godric Gryffindor
Professor Godric Gryffindor was a wizard of medieval times and one of the four founders of Hogwarts School of Witchcraft and Wizardry. He was good friends with Salazar Slytherin but opposed him when he later thought that they should not accept Muggle-bornstudents at Hogwarts. It is unknown when he died. Gryffindor's birthplace became known as Godric's Hollow, in his honour. Godric Gryffindor's portrait still remains at Hogwarts.
Helga Hufflepuff
Professor Helga Hufflepuff was a witch of medieval times and was one of the four founders of Hogwarts School of Witchcraft and Wizardry. While the other founders chose to have special students, Hufflepuff accepted them all, but the students of her house are described as loyal and hard-working. It is unknown when she died. Helga's portrait remains at Hogwarts
Albus Dmbledore
Professor Albus Percival Wulfric Brian Dumbledore, O.M. (First Class), Grand Sorc., D. Wiz., X.J.(sorc.), S. of Mag.Q.[7] (c. Summer[1], 1881[2]– 30 June, 1997) was the Transfiguration Professor, and later Headmaster of Hogwarts School of Witchcraft and Wizardry. Professor Dumbledore also served as Supreme Mugwump of the International Confederation of Wizards (?–1995) and Chief Warlock of the Wizengamot (?–1995; 1996-1997). He was a half-blood, Muggle-supporting wizard, the son of Percival and Kendra Dumbledore, and the elder brother of Aberforth and Ariana. His father died in Azkaban when Dumbledore was young, while his mother and sister were later accidentally killed. His early losses greatly affected him early on, even at his death, but in turn made him a better person. Albus Dumbledore was considered to be the most powerful wizard of his time. He was most famous for his defeat of Gellert Grindelwald, the discovery of the twelve uses of dragon's blood, and his work on alchemy with Nicolas Flamel. It was through Dumbledore that resistance to the rise of Lord Voldemort was formed, as it was he who founded and led both the first and second Order of the Phoenix. Due to the fact that he had a keen mind and legendary power, Dumbledore became the only wizard that Voldemort ever feared. He was the wielder and the master of the Elder Wand from 1945 until 1997 and considered by many as the greatest Headmaster to ever grace Hogwarts. As he was about to die by a cursed ring, he planned his own death with Severus Snape. He was killed by Snape during the Battle of the Astronomy Tower.