King of the Hellenes who reigned from 1913 to 1917 and from 1920 to 1922.
Konstantinos (English: Constantine) was the eldest son of King George I and Queen Olga of Greece. As Crown Prince, he married Princess Sophia of Prussia, a granddaughter of Queen Victoria and sister of Kaiser Wilhelm II, on 27 October 1889 in Athens. They had six children. All three of their sons ascended the Greek throne. Their eldest daughter Helen married Crown Prince Carol of Romania; their second daughter married the 4th Duke of Aosta; whilst their youngest child Princess Katherine married a British commoner.
He was commander-in-chief of the Hellenic Army during the unsuccessful Greco-Turkish War of 1897 and led the Greek forces during the successful Balkan Wars of 1912–1913, in which Greece expanded to include Thessaloniki, doubling in area and population. He succeeded to the throne of Greece on 18 March 1913, following his father’s assassination.
His disagreement with Eleftherios Venizelos over whether Greece should enter World War I led to the National Schism. Constantine forced Venizelos to resign twice, but in 1917 he left Greece, after threats of the Entente forces to bombard Athens; his second son, Alexander, became king. After Alexander’s death, Venizelos’ defeat in the 1920 legislative elections, and a plebiscite in favor of his return, Constantine was reinstated. He abdicated the throne for the second and last time in 1922, when Greece lost the Greco-Turkish War of 1919–1922, and was succeeded by his eldest son, Geórgios (George) II. Constantine died in exile four months later, at 1:30 AM on 11 January 1923, at Palermo, Sicily of heart failure.

Link to Wikipedia biography