Everyone has a special mark that the share with one other person- their soulmate. Denki was desperate to find his soulmate, but after failing to match the lightning bolt shaped mark on his wrist to anyone in the class, he became discouraged. As a member of Class 1-B you didn't see the electric user often, and neither of you were aware you shared the mark. One day during a joint training session you just so happened to catch a glimpse of the mark on his wrist.