Independent Greece was a monarchy until 1973: from 1832 to 1924 and from 1935 to ’73. There was a short republican interlude. For the first thirty years the king was a Wittelsbach: Otto. Afterwards the ruling house was Schleswig-Holstein-Sonderburg-Glücksburg, a Danish cadet branch of the house of Oldenburg.
Bavaria did not take part in the war of independence, when the supporters of Greece had been the UK, France and Russia. The Duke of Edinburgh is a Schleswig-Holstein-Sonderburg-Glücksburg.
Otto was the second son of the Bavarian philhellene Ludwig I. Ludwig and others lent Greece money to keep it going. (The eldest son, Maximillian II, succeeded his father in Bavaria and was the father of mad King Ludwig II.)
“‘You will certainly not want me to lose the larger part of my own capital because of having saved Greece,’ King Ludwig wrote in April  to his son Otto […].
‘Couldn’t you pay back the interest on the loan at least? … things are becoming impossible for your mother and your siblings.’”
From a piece by Alexandra Hudson at Reuters.
Otto loved his adopted country, but was not loved in return. Thomas Gallant quoted in Wikidepdia: he “was neither ruthless enough to be feared, nor compassionate enough to be loved, nor competent enough to be respected”.
He espoused the Great Idea (Μεγάλη Ιδέα), the dream, which died with the Treaty of Lausanne in 1923, of uniting all the Greek populations of the Ottoman Empire with Christian Greece and reestablishing Constantinople as the Greek capital.
He was styled King of Greece. His Oldenburg successors were Kings of the Hellenes.
Portrait by Gottlieb Bodmer
Portrait by Joseph Stieler