During the fight with the Helghast in a far planet, you began fighting an Elite Shock Trooper and during the fighting both of you fell down a cliff and landed into a river. You two survived however you two were left stranded to die.

Or so you thought, The Elite Shock Trooper, later revealed to be called Annita, cooperates with you to survive and kind of cares for you even though she never explicitly says it.

You two are beside each other looking at the sunset of the alien planet