Hadvar is an Imperial solider in The Elder Scrolls V: Skyrim. He is the nephew of Alvor and Sigrid and also a cousin of Alvor and Sigrid's daughter, Dorothy. He is one of the two soldiers that follow alongside the Dragonborn during the Skyrim Civil War, the other being Ralof of the Stormcloak Rebellion.
Hadvar and his Aunt and Uncle Alvor and Sigrid live in the small town of Riverwood just south of Whiterun. Alvor works as a blacksmith while his daughter (and Hadvar's cousin) Dorothy attempts to assist him.
It is also implied that he and Ralof actually knew each other before the war, as implied by a conversation they have before they retreat inside the nearby barracks.
Hadvar is first seen roll calling all the prisoners brought to Helgen to be executed. When he gets to the Dragonborn, he tells his captain that he does not recognize them and that they are not on the list. The captain tells him to forget about it and just send him to the block (despite the fact that the Dragonborn's crime isn't punishable by death). He offers his condolences better they get executed. Just when the Dragonborn is about to be executed, a black dragon flies in and destroys the town, killing most of its inhabitants. Hadvar, Ralof, General Tullius, Ulfric Stormcloak, and the Dragonborn all escape alive fortunately.
The Dragonborn can choose whether to follow him or Ralof into a nearby barracks and escape from it. Afterwards, he will tell the Dragonborn to meet him in Riverwood, where he tells his aunt and uncle about the dragon attack. They tell the Dragonborn to travel to the city of Whiterun and speak to the Jarl about the attack.
The Civil War will resume as a result of the dragon attack, if the Dragonborn chooses to side with the Imperials, they will have Hadvar as a constant companion during their missions, some of which include protecting Whiterun from the Stormcloaks, capturing an enemy wagon containing necessary supplies outside of Riften, and freeing Imperial prisoners from a fort in Winterhold. In between battles, and after the war ends, he can be found residing inside the tavern at Riverwood.