His friends would initially be shocked. Neville, for example, would look at Harry with a new - found respect. But as time goes on, they would start to expect more from Harry. They might think he can solve all their problems easily because of his power. This could lead to some friction in their relationships as Harry tries to explain that his power has limitations and he still needs their support as friends.
With Ginny, their relationship could be tested. Ginny might feel that Harry is becoming more distant as he delves into his new role. Harry, as the Master of Death, would have a different perspective on life and love. He might be more cautious about getting too close to Ginny, fearing for her safety. His friendships in general would be strained as he has to keep some secrets about his powers.
In some fanfictions, it makes Harry more confident. Since he knows he's the master of death, he feels a sense of purpose and power that he didn't have before. This newfound confidence allows him to take on Voldemort more fearlessly.
In a Master of Death Harry Fanfic, Harry's character development would be complex. He would have to grapple with the enormity of his new power. At first, he might be tempted to use it for revenge against those who wronged him in the past. But as he matures, he realizes that his power should be used for the greater good. He would also become more solitary as he has a power that few can understand, but he would still cherish his true friends even more.
He could fit as a powerful ally. His mastery over death gives him abilities that could enhance the Avengers' capabilities.
In some fanfics, like 'The Master of Death's Dominion', Harry uses his new status to become a benevolent dictator of sorts in the wizarding world, changing the political structure. In others such as 'Harry, the Reluctant Master of Death', he is hesitant to use his power and tries to hide it. And in 'Master of Death: A New Era', his being the Master of Death ushers in a new age of magic with different rules.
In some fanfictions, it changes by giving Harry more direct control over life and death. For example, he can revive the dead or prevent people from dying in a way that was not possible in the original books.
In crossover fanfics, the 'Harry Master of Death' concept often gets expanded. For example, in a crossover with the Star Wars universe, his mastery over death might be translated into an ability to resist the dark side's allure, which is related to death in a different way than in the Potter universe. He could use his power to save characters from the brink of death in a more 'Star Wars - y' way, like using the Force in conjunction with his magic.
His friends might be initially shocked. Hermione would be super curious and want to research all about it. Ron might be a bit jealous at first, but then come around. Their adventures would be different as Harry would have new insights from the founders.
Well, Harry's friends would have to adjust to his new role. Hermione might see it as an opportunity to learn about a whole new branch of magic. Ron, on the other hand, would probably be more interested in how Harry's powers could be useful in practical situations, like finding food in the forest. Their friendship would become deeper as they help Harry navigate his new identity as the God of Nature. They would also have to deal with the fact that Harry now has a different level of responsibility, and they would support him in fulfilling it.
His relationship with Hermione could become more complex. She might be both intrigued and a bit jealous of his new powers from the god inheritance. She would try to study and understand it, while also being worried about how it might change Harry.
One possibility could be that in this fanfiction, Harry Potter uses his mastery of death to have unique abilities in Skyrim. Maybe he can't truly die in the Skyrim world, and this gives him an edge in battles against dragons or other foes. He could also use his knowledge from the wizarding world to manipulate magic in Skyrim in ways that are different from the native mages.