King Achashverosh ruled over the whole of Persia. He was said to also ruled over 127 separate lands. We first see King Achashverosh at the beginning of the Megillah when he is throwing a massive party for all of the people he ruled over.
At one point he starts to boast to his men about how beautiful his wife Vashti is. He then calls Vashti to come and dance in front of him. She refuses and is beheaded.
Achashverosh is one of the central characters in the print story because everything seems to revolve around him. Not only did he marry Queen Esther but he also ruled over all of the lands and was able to control Haman.
What is also obvious is that he does like fairly decadent things for example he takes all the women often lands to be his wife what to hold a beauty contest. Another example is how he seems to be drinking through a lot of the story.