Someone who brings a legal complaint to a court is a suer. If you sue your next door neighbor for building a gazebo on your land, you are a suer.
When a person files a lawsuit against a company or another person, she sues, and can be described as a suer. The word is pronounced like sewer, though its meaning is completely different. The root of suer is the verb sue, which has its origins in the Latin word sequi, "to follow." Originally, to sue meant "to follow up in court."