Scar Meme!
I know I didn't make it pretty, but here it is:

These are all of Billy's scars, counting time on Stacy and from his homeworld. The one on his stomach stems from childhood, with an experiment in developing more efficient scissors. Its existence is headcanon, and I've taken it as the reason Billy never goes shirtless even after gaining confidence and working out with the other men on the team. Unfortunately, the behind-camera reason was probably far more insidious than that, but I don't want to speculate again.
The rough one on his shoulder is from a fight with a space pirate during the Nightmare King battle in March 2010, caused by heavy falling debris. The ones on his arm and leg are from a fight with the Ohm in June 2010, caused by their poisoned skin-slices. Most of his injuries don't end up this badly, but those were the few that stood out.

These are all of Billy's scars, counting time on Stacy and from his homeworld. The one on his stomach stems from childhood, with an experiment in developing more efficient scissors. Its existence is headcanon, and I've taken it as the reason Billy never goes shirtless even after gaining confidence and working out with the other men on the team. Unfortunately, the behind-camera reason was probably far more insidious than that, but I don't want to speculate again.
The rough one on his shoulder is from a fight with a space pirate during the Nightmare King battle in March 2010, caused by heavy falling debris. The ones on his arm and leg are from a fight with the Ohm in June 2010, caused by their poisoned skin-slices. Most of his injuries don't end up this badly, but those were the few that stood out.