Some history, largely from "Who's Who in Reigning Royalty
", William G Valko, 1969.
This is relatively old, and some of the spellings may have been Americanised, but it gives some useful background.
The Independent State of Samoa
House of Malietoa
This is the western region of what, in the nineteenth century was the Kingdom of Samoa. The Kingdom became the scene of rivalry between the Powers of Great Britain, Germany and the United States (the islands being eventually divided between German Western Samoa and American Eastern Samoa), as well as rivalry between the royal lines of Malietoa, Tupua, and Mata'afa.
The last monarchs from these royal lines before the partition of the islands were King Tupua Tanasese (vice-king from 1881-1887; King 1887-1888), King Mata'afa (reigned 1888-1889, and 1898; died 1912), and King Malietoa Tanumafili I (reigned 1898-1899).
With the defeat of Germany in WWI, Western Samoa passed as a Mandate of the League of Nations to New Zealand, but a Samoan nationalist movement soon arose to oppose NZ's control. The Samoan nationalist leader, Fautua (high chief) Tupua Tamasese Lealofi, was assassinated at Apia in 1929. New Zealand retained Western Samoa as a Trust Territory of the United Nations following WWII.
In September 1959 NZ's Parliament legislated giving the Trust Territory a cabinet government. Western Samoa overwhelmingly voted for independence in a plebiscite in 1961, and formally became an independent nation on January 1st 1962. It's constitution, which recalled the setup under German rule, provided for two joint chiefs of state, one representing the royal line of Tupua, and the other, the royal line of Malietoa (the position of Premier being in the hands at the time of a Mata'afa). When one chief of state dies, the other was to continue as sole Head of State, according to the provisions of the constitution. The survivor will be succeeded on his death by a Head appointed by the Samoan parliament for a 5 year term.
The Head of State (O le Ao o le Male) is addressed as His Highness
Current Head of State
His Highness Malietoa Tanumafili II, born 1913, Head of State from January 1 1962.
His Highness Tupua Tamasese Mea'ole (1905-1963).
In 1997 the name of the State changed from Western Samoa to Samoa, despite the protests of the American Samoans.