Scars can be interesting plot devices. A good backstory on a scar is the antagonist (if there is one) caused it, since the scar can serve as a motivator and reminder of the misdeeds the antagonist did, thus encouraging the protagonist to enact out revenge. Such as this:
I once encountered the dangerous and wanted mutant jaguar, named Rapier by the officials and peasants. Striking without warning, it has killed and devoured multiple townspeople, rising in infamy. It's only ever attacked near the crops which lie on the outskirts of town and around the plants growing out in the forest, which peasants used to regularly go out and harvest. It has also attacked at night; it seems that it's evolved to not need much sleep. The officials have placed a bounty on the creature; 10,000 gold coins. I couldn't decline the offer, no matter how dangerous the beast was; I researched and gathered as much information about the thing as I can before gathering my equipment and setting out to put it down once and for all. Little did I know it was two steps ahead of me. It ambushed me when I was waiting far-off, looking at a spot one of the townspeople warned me not to go to. It struck quick and hard, cutting deep into one of my calf. Fortunately, it left itself open to a counter-attack and I was able to swiftly cut into it with one of my knives. It recoiled and ran off to heal, and I made my way to town as fast as I could. I collasped before I could reach it, but one of the villagers found me and was able to carry me to a surgeon. After stitching up my calf, he charged a hefty sum but I was able to pay due to my previous successful completed bounties. Going to the tavern, I began to think up new ways to catch and kill that monstrosity, thinking of the new scar on my calf...
Certainly not a good example, but I hope it helps you in any way.