What Did Constantius II Really Look Like?

Google:
"Constantius 11 plaster bust"

Here is the description of Constantius II from Gore Vidal's novel, Julian (one of my favorites), when Julian first meets the emperor. It is fiction, but I think it does a good job:

Constantius was a man of overwhelming dignity. That was the most remarkable thing about him: even his most ordinary gestures seemed carefully rehearsed. Like the Emperor Augustus, he wore lifts in his sandals to make himself appear tall. He was clean-shaven, with large melancholy eyes. He had his father Constantine's large nose and thin, somewhat peevish mouth. The upper part of his body was impressively muscular but his legs were dwarfish. He wore the purple, a heavy robe which hung from shoulder to heel; on his head was a fillet of sliver set with pearls.

...

For an instant I was so close to Constantius that I could make out every pore in his face, which was sunburned dark as a Persian's. I notices the silkiness of his straight brown hair, only just beginning to turn gray. He was thirty-two, but I thought him ancient. I also remember thinking: what must it be like to be Emperor of Rome? to know that one's face on coins, on monuments, painted and sculptured, is known to all the world? And here-so close to me that I could feel the reciprocal warmth of his skin--was the original of that world-famous face, not bronze or marble but soft flesh and bone, like me, like any other man. And I wondered what IS it like to be the center of the world?

Later representations of the Roman emperors depicted them as they wanted to be seen.
Generally "pious and holy" with eyes looking upward and a rather benign look about them.
(Despite having usually killed off more than half the blood relatives !)

The illustration you posted is interesting. I am no expert on Constantius II but it seems a bit odd to me that the emperor is flanked by two horsemen that are certainly meant to represent Dacian warriors ! They both carry aloft the 'Draco' which was worshipped by the Dacians and was/is a national symbol.
I am unsure if the "pious" emperor Constantius II would have entered Rome with pagan banners flying beside his quadriga !
But I suppose stranger things have happened.