In some fanfictions, Batman's reaction could be more emotional than we usually see. He might be angry and feel a deep sense of hurt. This could lead him to be more reckless in his actions. However, he'd still rely on his training and intellect. He might also question his own judgment in trusting people, which could change his approach to future relationships within the Bat - family and with other allies.
Batman would probably isolate himself at first. He'd retreat to the Batcave to analyze the situation. He might feel a sense of guilt, thinking he should have seen the betrayal coming. After that, he'd start planning his counter - move. He'd use his vast resources and knowledge to expose the truth behind the betrayal and take down whoever is behind it.
He might isolate himself from the League at first. Batman is used to working alone, so he could retreat to his Batcave and plan his next move. He could also start to prepare counter - measures in case the situation escalates and the Justice League becomes a threat to him.
Batman might become more brooding. He could withdraw from the people around him, like Robin or Commissioner Gordon.
Batman would be heartbroken. He has trained Robin and sees him as a partner and almost like a son. He would first try to talk to Robin, to find out what made him turn evil. He would hope that he could reason with Robin and bring him back to the good side.
At first, Batman would be shocked and a bit confused. But quickly, his detective instincts would kick in. He would view the deaged Joker as a new and unexpected threat. He'd start gathering information about how the deaging occurred, in case it was part of a larger, more sinister plan. Batman would also be very careful not to underestimate the deaged Joker. He might try to isolate the Joker in a secure location to prevent him from causing chaos in Gotham. And he'd be constantly on the lookout for any signs of the Joker's old, devious self emerging from this new, younger form.
Some characters may be amused. They could find the creative and sometimes out - of - the - box ideas in the fanfiction entertaining.
Batman would probably first try to contain the situation. He'd use his resources as Bruce Wayne to spread misinformation or counter the revelation. He might also go on the offensive against Scarecrow to stop him from spreading the news further.
She could be devastated. Wonder Woman might go into a period of mourning, isolating herself from others as she grapples with the loss. She was used to having Batman as an ally, and his death would leave a big hole in her life.
In some fanfictions, Batman might actually show a moment of vulnerability. The kiss could stir up some long - buried emotions or simply catch him off - guard enough that he hesitates for a second. However, he would likely quickly suppress these feelings and return to his mission - focused state, but the kiss would still leave a mark on his thoughts about Harley Quinn.
The Justice League as a whole would have a diverse range of reactions. Some members like the Flash would be all for it, seeing it as a fun exploration of Batman. Others like Wonder Woman might use it as a way to understand how the public perceives Batman's role in the League. And then there are those like Aquaman who might just shrug it off unless it directly affects his interests. The fanfiction could also potentially create some internal discussions within the League about how they are all portrayed in the public's imagination.
One common plot twist could be Batman being betrayed by a close ally like Robin. Maybe Robin gets brainwashed by a powerful villain and turns against Batman. Another could be Batman's trusted butler Alfred being forced to betray him under some extreme threat. For example, a villain might hold Alfred's family hostage and make him leak Batman's secrets.