Given that in my game, all kinds of people tied themselves to a player that didn't show up, it's not a big deal. They still have connections through the absent third party.
Also the early scenes might have some connection with the absent player, based on what some of the PCs have learned so far.
Let the guest star in any story, or with any interesting NPC that has been important in the stories. Somebody getting hospitalized, for example, is a great excuse for a PC relative to show up...(or I guess killed too...at the funeral instead of the hospital waiting room)
And yeah. They can develop aspects in play too....but aspects actually do help tie in newbies to an established group, so it's helpful to the GM if the players make at least a little effort along those lines.