How's this work, exactly? At the beginning of a mission the Imperial player can add a single Imperial and Scum unique named character to his reserved deck?

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At the beginning of a campaign when you gain the 0XP class cards, you choose one villain of each affiliation to gain for the duration of the campaign.

See the existing threads for more.

You need to earn villains before you can add them to open group. One of the starter cards says you earn an imperial and a scum villain at the start of the campaign, so you'll be free to add them to your open groups (not reserved). Then, the second part of the card says you add one earned villain (either from that card, or from any reward from agenda missions) to your open groups during each setup. That way, you always have a villain in your open groups.

Reserved cards are mission specific and cannot be used during optional deployment, they are deployment used during mission triggers. Sometimes, there are villains in those reserved groups. Open groups are deployment cards the imperial player chooses at the start of the mission and can be deployed during an optional deployment.