Short answers, pulled from the story.
The word Varangian comes from the Old Norse term væringi, which combines vár meaning pledge or faith and gengi meaning companion. This compound originally described a sworn companion or confederate who had taken service with a new lord through a treaty of fealty.
According to the 12th-century Primary Chronicle a group of Varangians known as the Rus settled in Novgorod in 862 under the leadership of Rurik. Before Rurik the Rus might have ruled an earlier hypothetical polity known as the Rus Khaganate.
Initially the guard was composed of Varangians who came from Kievan Rus. Immigrants from Scandinavia predominantly immigrants from Sweden but also elements from Denmark and Norway kept an almost entirely Norse cast to the organization until the late 11th century.
There are raised stone memorials called runestones throughout Scandinavia of which almost all are found in Sweden. These Varangian runestones commemorate various fallen warriors through carved runes and mention voyages to the East or the Eastern route or to more specific eastern locations such as Garðaríki what is today Russia and Ukraine.
By the late 13th century Varangians were mostly ethnically assimilated by Byzines though the guard operated until at least the mid-14th century and in 1400 there were still some people identifying themselves as Varangians in Constantinople.