The Varangian guard is so interesting and could have been something out of a book or movie. A Roman emperor having an elite squad of viking warriors to protect them!
In the sources they show up just as often fighting in battles where the emperor was there and when he was not.
The did protect him on campaign, but that might just be because the were the primary infantry arm.
Some people think of them like the Praetorian guard, but that isn't really accurate.