WARNING: The following article contains major spoilers for Amazing Spider-Man #800
Flash Thompson was there at the beginning.
Introduced in the legendary Amazing Fantasy #15 as a representation of the popular teenagers that belittled the studious Peter Parker, Flash slowly grew into an important part of Peter's supporting cast. Initially functioning primarily as an antagonist, as the characters progressed beyond High School Flash's role in the books changed and evolved. Whether as school bully, Spider super-fan, soldier, screwup or superhero, Flash played an important role in the books, but one that it's sometimes easy to overlook in favor of more well-known characters such as Harry Osborn or Mary Jane. With Flash's heroic sacrifice in Amazing Spider-Man #800 appearing to be the end of his hero's journey - at least for the moment - it seems an appropriate time to look over Flash's history and examine how a stereotypical Jock evolved into someone who selflessly made the ultimate sacrifice.
In the Stan Lee/Steve Ditko era of Amazing Spider-Man, it's fair to say that Flash wasn't the most sympathetic of characters. The big man on campus, his role was as the High School antagonist to Peter, causing problems in his personal life to match those in his costumed one. But Lee and Ditko came up with an ingenious twist: Flash Thompson may have thought that Peter Parker was a useless bookworm, but he idolized Spider-Man. So much so that he was president of the Spider-Man fan club! This hero worship often saw Flash unwittingly aid Peter, while also helping show readers that his character did have some redeeming features.
When Peter went to Empire State University and his social circle expanded, the introduction of characters such as Harry Osborn, Gwen Stacy and Mary Jane Watson saw some of the focus shift away from Flash. While he was part of Peter's wider friendship group this was primarily through their shared connection with other characters. The relationship between the two was still primarily defined by putdowns, bad feeling and resentment. And when the book took a more soap-opera turn under the team of Stan Lee and John Romita, more often than not the cause of the bad blood was conflict over women.
Where Flash's character first began to see real growth is when he dropped out of University to join the US Army, fighting in the Vietnam War. This showcased a more selfless side behind the bluster, leading to a grudging respect between him and Peter. For the next few years he would make occasional appearances in Amazing Spider-Man between tours of duty, before finally leaving the Army and trying to find a role for himself in civilian life.
The improved relationship between Peter and Flash from this point on can be seen in a development that would have been unthinkable only a few years ago. After Harry Osborn became the Green Goblin and his and Peter's apartment was destroyed, Peter and Flash became roommates.