If Percy joins the Titans, his allies could be some of the lesser - known Titans who are tired of the constant fighting. They might see Percy as a way to bring about change. Enemies? Well, the Titans who are fully committed to overthrowing the Olympians and establishing their own tyrannical rule would be against him. Also, Poseidon, his father, might be conflicted about Percy's choice and could end up being an enemy if Percy doesn't change his course.
As for enemies, Percy could encounter the Titans who were once banished there. They might see Percy as an intruder and a threat, and try to eliminate him. Also, the monstrous creatures that lurk in Tartarus, like the Empousai or the Chimera, could be his foes.
Well, perhaps Percy could be seeking a new kind of power or a different way to prove himself. He might initially be lured in by the Titans' promises of a more 'free' and unconstrained way of using his abilities. Maybe he gets disillusioned with the Olympians and thinks the Titans offer a better alternative.
His old friends from Camp Half - Blood like Annabeth could be his allies. She would support him through the transition of becoming a god. Grover too, as his long - time friend, would be there for him. Also, some of the minor gods who have always been on good terms with Percy, like Hestia, might offer their help and guidance as he steps into his new role as a god.
Some of the Greek heroes like Achilles could be Percy's allies. Percy might learn a lot from Achilles about combat and heroism. And there could be some new characters created specifically for this fanfiction, like a young seer who can predict the outcome of battles and helps Percy make the right decisions.
Some of the more traditional and rule - following demigod groups at Camp Half - Blood could be against him. They believe in following the established ways and Percy's rogue behavior goes against that. They might see him as a troublemaker and try to bring him back to the 'right' path.
There might also be a lost spirit or a demigod who was previously trapped in Tartarus and has managed to survive. This character could share their experiences and skills with Percy, and together they could try to find a way out or at least make the best of their situation in Tartarus.
Annabeth could be a great ally. Her intelligence and strategic thinking would be very useful in dealing with the chaos Percy often gets into. She could help him plan his moves and figure out the motives of chaotic enemies.
Annabeth would definitely be a great ally. Her strategic mind could help Percy navigate the complex situations that come with his Titan heritage. She could also research about Titans and find ways to counter any threats that target Percy because of his parentage.
It could be a senior officer who has a grudge against Percy for some reason. Maybe Percy accidentally made him look bad in front of the higher - ups early on.
There could be a group of anti - military rebels outside the military. They see the military as a tool of oppression and are constantly causing trouble. Percy might have to deal with them, either through combat or trying to negotiate and understand their point of view.