A champion concept.
13-37 is a ranged (550 range) champion who excel at dealing with lone targets. He can also switch between 2 forms, so I recommed you to read his E first.
There is nothing really known about 13-37. His was first seen near Piltover, where he came in a strange-looking portal. He seemed to wear technological outfits and weapons, but none saw something similar before. After few seconds, he began without explanation to kill what was near him, with a unknown firepower. After hours and about 40 victims, he was captured, and then questioned. He said he was a soldier, sent to "pacify and send the orders", and was named "13-37", he didn't want to say anything about his origins, how he came here and what kind of weapons he use.
After several days of imprisonment, he was send the League, because he was simply too dangerous to be released, and he would probably not return to where he come.
"The League is the only way to keep him from killing people."― One of the many guards that hold him
NOTE : date is written Day/Month/Year