I think you have to use an escort objective and set it to 'required' for that.
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
And spawn an ambush triggered on the rescue.
Janrith
Triumph Heroes:
=============
Saunik: lvl 50 Mutant Sonic/Electric Blaster
Photaun: lvl 50 Peacebringer
Taukezo Kensei: lvl 50 Natural Willpower/Dual Blades Tank
Published Arcs:
7326 Avenging the Dead
135096 Superhero Downtime
Oh also, can't you make the rescuee a combatant as opposed to passive so they can be attacked?
Janrith
Triumph Heroes:
=============
Saunik: lvl 50 Mutant Sonic/Electric Blaster
Photaun: lvl 50 Peacebringer
Taukezo Kensei: lvl 50 Natural Willpower/Dual Blades Tank
Published Arcs:
7326 Avenging the Dead
135096 Superhero Downtime
I believe "keeping someone alive" arcs have been done to death. 
(j/k) Good luck with yours. 
You would set it as an escort that is required for mission completion, and set the rescuee as passive so he or she could not attack, but be attacked.
Another question: does anyone know if you set them to passive, will they use defensive powers, like armor toggles?
Ah I was thinking passive vs. Non-Combatant...
Janrith
Triumph Heroes:
=============
Saunik: lvl 50 Mutant Sonic/Electric Blaster
Photaun: lvl 50 Peacebringer
Taukezo Kensei: lvl 50 Natural Willpower/Dual Blades Tank
Published Arcs:
7326 Avenging the Dead
135096 Superhero Downtime
I'd like to have a mission where you keep an ally alive as part of the mission goal. Have not yet figured out how to set it such. Any tips in how to do it?