No, there isn't a stat for the loser in a hand as that isn't a single person in most cases.
EDIT: Now that I read your title again, if you mean you want to look at a specific player then you can just add that player to the Player list and select them, and then you can filter everything from their point of view instead of your own.
If you play heads-up then you do this with a Hand report in My Reports
Click the "Filters" link and choose 'expression filters' and turn off the 'Filter on Active Player' option. This will mean that you see hands from all players' point of view.
You can then add a filter for winning the hand (and whatever other criteria you like).
If you also add a filter for "Player is Hero" and 'NOT' that filter
then you will see the hand from the other player's point of view (ie. hands you lost, but actually filtered for the villain winning instead).
Then you can just use the normal "Player" stat to see the winner's name.