It could be an old enemy of Sirius Black, someone from his past like a Death Eater he had a personal vendetta with. This Death Eater could have found a way to influence Sirius's actions and make him kick out the Order, with the aim of destroying the resistance against Voldemort, making the Death Eater the main antagonist.
One possible reason could be miscommunication. Maybe Harry does something that Sirius misinterprets as a betrayal or a stupid mistake, leading to Sirius lashing out.
Well, it's a rather unusual concept for fanfiction. Usually, Sirius Black is more of a protector or mentor figure to Harry Potter. However, if you search on platforms where Potter fanfictions are hosted, you might find some that explore this idea in a creative way. Maybe it's part of an alternate universe scenario where their relationship takes a different turn for some plot - driven reasons.
It could be Bellatrix Lestrange. Since she is a Death Eater and has a personal vendetta against Sirius, she would be a great antagonist. She might try to kill Sirius again and also target Harry to please Voldemort.
In fanfiction about Sirius Black in the Order of the Phoenix, one common theme is his struggle with the restrictions placed on him. He's a free spirit, but the Order has its rules. Some fanfictions might depict him chafing against these rules, while still trying to do what's right for the cause. His connection to the Black family legacy is also an area of exploration. How does he reconcile his hatred for his family's dark past with the fact that he's still a Black? Fanfiction can provide various interpretations, showing his internal conflict and how he overcomes it to be a valuable member of the Order.
One of my favorite scenes in Sirius Black Order of the Phoenix fanfiction is when he finally gets to have a proper heart - to - heart with Harry. It shows his vulnerability and how much he cares for Harry. Another great scene is when he uses his animagus form to outwit the Death Eaters and save the day. It really showcases his cleverness and his willingness to put himself in danger for the greater good.
It could be that there is no traditional 'antagonist' in the sense. Maybe it's just the situation and the conflict between Harry and Ron's personalities. But if we had to consider one, it could be an outside force that is influencing Ron's actions and making Harry distrust him. For example, a new dark magic object that is affecting Ron's judgment.
It could be a new character introduced in the fanfiction. Maybe someone who has infiltrated the Order and wants to create chaos by turning them against Hermione. This person could be a spy from the Death Eaters or another dark - aligned group, with the goal of weakening the Order from within by making them lose one of their most intelligent and powerful members.
Voldemort himself could be the main antagonist in this scenario. He might offer Hermione something she desperately wants in exchange for her betrayal. Maybe he promises to spare her Muggle - born friends if she gives up the Order's plans and secrets. His powerful and intimidating presence could easily force Hermione into a difficult situation where she feels she has no choice but to betray.
Since I don't know the details of this '23 kicks fanfiction', I can't say for sure who the main characters are. They could be original characters created by the author specifically for this story, or they might be characters from an existing fandom that the fanfiction is based on and have some connection to the '23 kicks' concept.
It could be a rogue Death Eater who escaped justice and is now seeking revenge on the Order and anyone related to them. So, Teddy, being Lupin's son, would be a target.