I have a baseless idea about who the other person Bertholdt grabbed could be. As I've said before it is probably one of those characters who have had quite a few lines in the past, but that you wouldn't remember in the long run. And my choice in this case, considering
what little is seen of the character's hair (seemingly blonde, middle length), and one appearance a little ago in the front lines, is this character:
Rico, Squad Leader of the 1st Division Force of the Stationary Troops, and the sole survivor of her division in the mission to have Eren block the wall with the giant rock.
Rico - Shingeki no Kyojin Wiki
When I was thinking the other day of who it could be that Bertholdt grabbed she popped in my mind, and today I was reading the 3rd volume (just bought it!), and she was around for quite a bit.
As I say, this is absolutely baseless, and I only propose it because of her hairstyle, the fact that she fits the profile of a character who's been around without making much of an impression, and has notable skills like the rest of Titans hidden amongst mankind so far.