The healer could be an old, wise wizard. Wizards are often depicted as having great knowledge and magical abilities in many fictional universes. This wizard could have seen the horrors of the war and decided to use his powers to heal those in need, like Nico.
Perhaps it's a young, inexperienced but extremely empathetic healer. This person might not have the most powerful magic but has a natural gift for understanding others' pain. They are drawn to Nico's case and through trial and error, they manage to find ways to heal him. It could be an interesting story of growth for both the healer and Nico.
It could be another war veteran. This person has also been through the trauma of the war but has somehow found a way to heal themselves. They recognize the same pain in Nico and use their own experiences and new - found skills (maybe they learned some form of self - healing magic during the war) to help Nico. This would add a layer of realism and connection between the two characters.
One possible plot could be that Nico is physically and mentally scarred from the war. The healer, perhaps a kind-hearted magic user, uses their powers to slowly mend Nico's physical wounds. They also take the time to listen to Nico's traumatic war experiences, providing emotional support. As time passes, Nico starts to regain his confidence and strength, and they form a deep bond through this healing process.
It could be some remaining monsters that survived the giant war and are now seeking revenge. They might target Nico specifically because of his role in the war. Another possibility is some rogue demigods who didn't like the outcome of the war and blame Nico for it.
Well, one possibility could be Nico dealing with the aftermath of the war. Maybe he's trying to find his place among the survivors. He might be struggling with PTSD from the battles he faced. Another could be him forming new relationships or strengthening old ones. For example, he could become closer to his friends from Camp Half - Blood.
The camp leaders might betray them. If there is a power struggle within the camp, and Percy and Nico are seen as threats to the current leadership's power, the leaders could set them up. They could make it look like Percy and Nico are breaking camp rules or endangering the other campers, which would lead to the rest of the camp turning against them.
One possibility is that they encounter other lost souls in Tartarus. These could be the souls of ancient heroes or villains. Interacting with these souls could either help them or put them in more danger. For example, some souls might offer them advice on how to navigate the treacherous terrain of Tartarus, while others might try to deceive them into making wrong decisions. And they would surely be tested both physically and mentally. The harsh environment of Tartarus would push their limits, and they would have to overcome their inner fears and doubts to have any chance of escaping.
Neville Longbottom could also be considered a natural healer. He has a great affinity for Herbology. His skills in growing and understanding plants could easily translate into using them for healing. He could be the one to discover new uses for common and rare magical plants alike, perhaps creating poultices or potions from them to heal various ailments that the regular wizarding medical field hasn't fully explored yet.
It could be a jealous demigod. Maybe someone who is envious of Percy's popularity and Nico's unique powers. This person might see an opportunity to get rid of them and take their place in the demigod hierarchy.
Another option could be a new demigod. A character who was not very involved in the war but is now thrust into the post - war chaos. This new character could bring a fresh perspective and could be learning about the demigod world and its history while also trying to help in the rebuilding and healing process.
A rogue ninja from a formerly warring village could be the main antagonist. This ninja might hold a grudge against the new peace and wants to disrupt it. He could be powerful and have his own group of followers who are also discontent with the current situation.
Some former Death Eaters who escaped justice could be the main antagonists. They might be plotting to bring back Voldemort's ideology in a more covert way. Another possibility could be new dark wizards who emerge, seeing the post - war chaos as an opportunity to gain power.