A rebel leader, Northumberland is a member of the Percy family and the father of the recently slain Hotspur. In Richard II Northumberland played a major role helping King Henry IV to the crown but he became disgruntled with the monarch in Henry IV Part 1, where Northumberland participated in the rebel plot to overthrow Henry. However, Northumberland ended up phoning in sick to the battle at Shrewsbury, leaving his son, Hotspur, vulnerable to the king's powerful army.
In the opening scene of Henry IV Part 2 Northumberland, who has been "crafty sick" (meaning he's a big old faker), receives inaccurate news that Hotspur has been victorious at Shrewsbury. Shortly thereafter, he learns the devastating truth: his son has been killed in man-to-man combat by Prince Hal. The news elicits a strong response from Northumberland, who makes a sudden and miraculous recovery from his "illness" as he throws down his crutch and calls for apocalyptic civil warfare against the king: "Let order die! […] But let the spirit of the first-born Cain / Reign in all bosoms" (1.1.170; 173-174).
It turns out, though, that Northumberland is all talk and no action, just like he was in Part 1. In Part 2, Northumberland promises to join the Archbishop of York's rebellion against King Henry but then backs out at the last moment. Not only is he a no-show at the show-down at Gaultree Forest, he also sends one of his famous "sorry but I can't make it to the party" letters.
What changes Northumberland's mind about getting his battle on this time around? A major guilt trip (compliments of his wife and daughter-in-law), that's what. (Though, we're definitely not ruling out Northumberland's tendency toward selfish self-preservation.) Lady Percy, Hotspur's widow, does quite a number on her father-in-law, convincing him to abandon his plans to go to war by reminding him that he didn't seem to have a problem staying home from the battle at Shrewsbury, even though he promised his son he would be there to support him. "Who then persuaded you to stay home" she asks just before she says "There were two honors lost, yours and your son's" (2.3.15, 16). In other words, Northumberland's betrayal of his son is the reason Hotspur's dead. As in Henry IV Part 1, Shakespeare reminds us, via Northumberland, that fathers can't always be trusted to care for and protect their children. Check out the theme of "Family" if you want to know more about this.