Kronos was a Titan in Greek mythology. He overthrew his father Uranus. Kronos was afraid that one of his children would overthrow him as he did to his father. So, he swallowed his children as soon as they were born. However, Rhea, his wife, tricked him when Zeus was born. She gave Kronos a stone wrapped in cloth instead of Zeus. Zeus grew up and later waged a war against Kronos and the Titans, eventually defeating them and imprisoning Kronos in Tartarus.
Kronos' story is full of big events. First, he took power from his father. That was a huge shift in power. After that, he started swallowing his children. This was a really cruel act. But the most important event was when Zeus came back to fight him. Their battle decided who would rule the cosmos. Zeus won and Kronos was sent to Tartarus.
Perhaps there are some traitors from the other side. These characters, seeing Kronos' power and potential for victory, switch sides and help him in the war. They might provide valuable information about the enemy's defenses and plans.
In many fanfictions, Percy might use his water powers in a unique way. Maybe he combines his control over water with his sword fighting skills. He could create a giant water vortex to disorient Kronos and then strike with Riptide while Kronos is off - balance.
Some of Kronos' minions, like the monsters he commands, could be antagonists. For instance, the dracaenae. They are always eager to prove themselves to Kronos and might see Percy as competition within Kronos' ranks. They could constantly try to undermine Percy's position and cause trouble for him.
The other Titans could be main antagonists. For example, Hyperion who is known for his great strength and his desire for destruction. He might not fully trust Percy and constantly try to undermine him in Kronos' eyes. Another could be Atlas, who is already a major foe in the Percy Jackson series. He might see Percy as a threat to his own plans within Kronos' regime.
Maybe there are some original characters created by the fanfiction author. These characters could have their own unique abilities and backstories. They might be the ones who tip the scales in Percy's favor, for instance, by having a special power that weakens Kronos just before Percy makes the final blow.
Luke could also be a prominent character. Since he has a complex relationship with Kronos in the original story, in fanfiction he could play a role in either helping or hindering fem Percy's efforts against Kronos. Nico di Angelo is another possibility. His powers over the dead could be useful in a fight against Kronos, and he might team up with fem Percy for that reason.
To develop the character, start with his backstory. Since he's the son of Kronos, he might have grown up in a very different environment compared to other demigods. Maybe he was hidden away by a minor god who felt sorry for him. His past experiences of being alone and shunned because of his father would shape his personality, making him a bit of a loner at first.
Well, in the Kronos origin story, Kronos is a significant figure. He emerged in the context of the Greek mythological cosmos. As I mentioned before, his actions were driven by power and the fear of being overthrown. His relationship with his family members, like his parents Uranus and Gaia, and his own children, was complex. His reign was marked by this cycle of fear and the attempts to prevent his downfall. And it was Zeus who finally broke this cycle and brought about a new order in the Greek mythological world.