Two faces familiar from the series are no longer aboard, but we meet them later: the "companion"
what does series mean & aboard on what?
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Series refers to television series
A group of episodes of a television program broadcast in regular intervals with a long break between each group, usually with one year between the beginning of each.
Specifically, it refers to the Firefly tv series.
: on, onto, or within a train, a ship, an airplane, etc.
So the two people are no longer riding in the ship. They were aboard the spaceship Serenity.