Rex's text:
"Reaction: After this unit is focused to strike, it gains X different combat icons of your choice until the end of the strike. X is the number of different affiliations among cards you control."
From the rules for Mission cards, we understand that when you play a mission card, that card is under the control of your opponent. So, if the DS plays a non-neutral mission against the LS, such as Prepared Ambush, Rex could count that as a one of the different affiliations under LS's control?