Wonder Woman is one of the bright spots of Justice League. Even with all of the gloom and doom surrounding the disastrous box office performance of what was meant to be Warner Bros.' crown jewel DC superhero team up, and the growing demand to see Zack Snyder's completed director's cut, Justice League does have its virtues. Chief among them is Wonder Woman (Gal Gadot).
By the end of Justice League, Diana Prince is not just one of the public faces of the new super team defending the world, but has also replaced Batman as the League's leader. This neatly completes Diana's macro arc in the DCEU from sheltered but courageous Amazon, to war hero, to recluse for nearly a century, to her reemergence as a superhero and now embracing a public identity as Wonder Woman.
Though fans met Diana Prince in Batman V Superman: Dawn of Justice while she was in the middle of her journey, Patty Jenkins' Wonder Woman did the yeoman's work establishing who Diana is and how she came to be. The only child born on Themyscira, Diana was kept unaware by her mother Queen Hippolyta (Connie Nielsen) that she was not only the daughter of Zeus but she was also the fabled Godkiller destined to kill Ares (David Thewlis). Diana was meant for greater things than remaining on Paradise Island; she was meant to save and protect the whole world, though it took her decades to fully embrace her ultimate mission.
First, Diana had to learn that humanity is neither purely good nor purely bad, as her volumes of ancient texts led her to believe. When she joined Steve Trevor (Chris Pine) to fight in World War I, she naively argued that the flaws in humanity aren't inherent but the direct cause of Ares' influence. Her experience in the Great War was a coming of age for Diana. By killing Ares, she entered the full bloom of her godly powers, but it came at a tragic cost. She watched Steve make a heroic sacrifice to help end the war. He was a spy who died a hero; more importantly, despite his flaws, he was a good man that Diana got to spend too short a time with. Losing Steve affected Diana so deeply that she decided to remain in the shadows for the better part of the 20th century.
Someone who knows all about remaining in the shadows is the Batman (Ben Affleck), the second person who got a sense that this beautiful but guarded art dealer from the Louvre is more than she seemed to be. The first was Lex Luthor (Jesse Eisenberg), who acquired a 1918 photo of Diana and her World War I friends taken in Veld, Belgium after their first victory in World War I. Diana was one of four metahumans Luthor had begun keeping files on, but it was Batman who was able to form a tenuous connection with Diana, albeit by having the files he stole from Luthor swiped (then returned) by Diana.
Diana reemerged as Wonder Woman to stand alongside Batman and Superman (Henry Cavill) against Doomsday, but after Superman died destroying the monster, Diana walked away from Batman's hopes that they find others like them and form a team to stop the alien invasion to come. Diana still wasn't ready to embrace being Wonder Woman and step into the light. However, Bruce Wayne wouldn't give up; he knew with Superman gone he desperately needed Diana. The modern day framing sequences of Wonder Woman saw Bruce send Diana a present: the original copy of the 1918 photo. He hoped this would stir the hero inside Diana to once again become Wonder Woman. It was a bit manipulative (as is Batman's way), but pushing Diana's Steve Trevor button worked. By the end of Wonder Woman, Diana reaffirmed her commitment to protecting the world as its hero.
When Batman began assembling the Justice League to fight Steppenwolf's Parademon invasion, the first person he naturally reached out to was Diana. She was already made aware that the invasion was imminent thanks to the Amazons, who lit the Flame of Athena to inform her they had lost their Mother Box to Steppenwolf. Though Diana's role in recruitment was to bring Cyborg (Ray Fisher) into the League, Batman really had grander plans for her: he wanted Wonder Woman to lead the Justice League.