Annabeth could be a likely candidate. She's always been by Percy's side, and she'd stop at nothing to save him. Her intelligence and combat skills would be useful in the dangerous environment of Tartarus.
Another possibility is Grover. He's Percy's loyal friend, and his connection to nature might give him an edge in Tartarus. He could use his nature - related powers to navigate the dark and dangerous realm and find Percy.
Annabeth might try to find a way to get to him. Their relationship is very strong, and she's not one to just leave Percy in danger. She could use her intelligence to figure out a way to enter Tartarus and help him escape.
His friends like Annabeth or Grover could be the ones to rescue him. They might discover a way into Tartarus and brave all the dangers to get to Percy.
It's possible that an unexpected character like a friendly Titan could rescue Percy. There could be a Titan who has had a change of heart and realizes that Percy is important for the balance of the world. This Titan might use its own knowledge of Tartarus and its powers to free Percy from the clutches of that terrifying place.
Chiron, the centaur mentor at Camp Half - Blood, might get involved in the rescue. He has a vast knowledge of the demigod world and could use his wisdom and connections to track down Luke and free Percy. Also, some of the other demigods at the camp who look up to Percy might volunteer to help, like Clarisse. She may not like Percy much, but she wouldn't want a fellow demigod to be in the hands of Luke.
Annabeth could be the one to come to Percy's rescue. She is very intelligent and resourceful. She might figure out Gaea's weaknesses and use that knowledge to free Percy.
He would surely interact with Annabeth. Their relationship is central in the Percy Jackson series, so in a Tartarus fanfic, she'd likely be there to support him or be in a similar situation as him.
Nico di Angelo could be involved. Since he has some knowledge of the Underworld and Tartarus, he could play a crucial role. He might be the one to guide Annabeth or other rescuers to the right places in Tartarus to find Percy. Also, some of the Titans or primordial deities who have a connection to Tartarus could be important characters, either as obstacles or potential allies for Percy.
There's no way to tell who the author is just from the title 'percy is tartarus fanfiction'. It could be written by any fan of the Percy Jackson series who wanted to explore this particular scenario in their own writing. It might be someone new to fanfiction writing or an experienced writer in the Percy Jackson fan community.
Annabeth would most likely be the first one to try to rescue Percy. She and Percy have a deep connection, and she wouldn't stand by while he's in danger. She's smart and resourceful, so she'd come up with a plan to find out where Aphrodite has taken him and then figure out a way to get him back.
He might rely on his powers. Percy is a son of Poseidon, so his control over water could be crucial. He could use water sources in Tartarus, if any, to heal himself, create barriers, or even find a way to escape. For example, he could manipulate the Styx water to his advantage.