Wikipedia sometimes uses the term VIP (i.e. "very important person") for this role, and "principal" as a generalization of a principal-agent relationship is also sometimes used. "Client" is another common modern usage.
The trouble with all of these, however, is that they are modern usages and you are seeking a more archaic feeling term.
"Beneficiary" might also be appropriate by analogy to a trustee and beneficiary, or executor and beneficiary.
The term liege is used for a man at arms protecting a "Master" or "Lord" but those terms imply more permanency than this relationship and implies a subordination of the protection provider that doesn't seem to be present in this relationship, although "master" and "servant" once had a broader and less pejorative sense than they do today.
Also, "master" is often how someone caring for a child in a formal setting will refer to a male child for whom he is responsible, even though he isn't truly the servant of that child, as opposed to the child's parent or guardian.
The legal term for a "guest" in a ship or an inn is "invitee" but that isn't colloquial enough.
The best example I've come up with is the word "ward" by analogy to a guardian-ward relationship, as a bodyguard who provides protection and guardian are somewhat analogous, despite the fact that "ward" suggests that the ward is subordinate in authority to the guardian despite being higher in priority (as a fiduciary like a guardian must but the interests of the person owed the duty higher than his own).
Another word that doesn't have too much baggage and doesn't sound too modern is "charge" (as in a nanny or babysitter's "charge"). This usage is derivate of the use of the word "charge" in the sense of an "order" or "to take charge", a "charge" is the object of the order you have been given to protect someone. This word puts emphasis on the obligation rather than the relative status of the parties.