"'he came in second in our school compeition laster year, only after his brother,'"
-Katniss talking about Jackie in "The Hunger Games,"
Jackie Mellark is the older, taunting brother of Peeta Mellark, a victor of District 12. He is also the younger sibling of Logan Mellark. He is the son of Zander Mellark. He is abused by his mother, but Peeta gets the worst of the abuse from the mother.
When Peeta is reaped, Jackie chose not to volunteer, even though he stood a fair chance of winning, since he won a wrestling competition, with Peeta coming in second. Logan didn't volunteer since he was too old to, being 20 years old, while Jackie still could at the age of 18. This left his relationship with Peeta bitter.
When Peeta is reaped again, he thinks Peeta has a good chance of winning, since he has experience from the previous Games. 34 mins after the Games end, the Capitol bombs 12, sending everything into flames. The Capitol also bombs the bakery, while Jackie was still in it. After 4 hours of trying to escape, he dies anyway by a second bomb.