Oliver Queen is arguably the hero who started it all at this point. Following the success of Arrow, executive producer Greg Berlanti convinced DC Entertainment, Warner Bros. TV and The CW to roll the dice on the expensive and effects-heavy The Flash, which not only expanded the DC lineup in primetime but also fundamentally altered the chemistry of Arrow forever.
Oliver Queen, after five years on a desert island (ish) during which he taught himself survival, combat, and archery skills, returned to Star(ling) City with a plan to clean up the streets and make it safe for everyday people.
Shipwrecked on the same boat that landed Oliver Queen on Lian Yu, Sara Lance was apparently drowned, but in fact found herself washed up on shore elsewhere, and was rescued by the League of Assassins.
After recovery, and falling in love with the daughter of Ra’s al Ghul, Sara trained with the League for 5 years before returning to Starling City herself. She died again, and had to be revived with a little help from a master of the mystic arts, at which point she joined a time-traveling team of costumed adventurers who would come to call themselves the Legends.