Physically, he might become more rugged and less concerned with his appearance. Without the trappings of wizarding society, he may grow a beard or let his hair get long and unkempt. Mentally, he could become more introspective. He has time to think about all that has happened in his life, and this could lead to a deeper understanding of himself and the events that shaped him.
His life would change drastically. He'd be living in a different environment, with different values being pushed on him. Instead of the Dursleys' neglect, he'd face Lucius's attempts to mold him into a pure - blood - ideal wizard.
It could change his life by giving him a new form of magic. For example, he might be able to combine the power of the stand with his existing magic, making his spells more powerful.
One possible reason could be to escape the fame and the constant scrutiny that comes with being 'The Boy Who Lived'. After all the battles and chaos, he might just want some peace and quiet to heal his mental and emotional wounds.
If the device in the fanfiction is a time - traveling gadget, it could completely transform Harry's life. He could go back in time to learn from the founders of Hogwarts, understand the origin of certain dark magic, and use this knowledge to prevent future disasters in his own time. It would also give him a chance to meet his parents and gain a deeper understanding of his own heritage.
The phoenix could give Harry a sense of purpose. Knowing he has such a powerful and magical creature by his side, he might feel more confident. For instance, he could be more daring in his adventures and face his enemies without as much fear.
It makes him seem more self - centered at first glance. Traditionally, Harry was seen as the savior of Britain, so leaving it to its fate goes against that heroic image. But on a deeper level, it could also show his growth into a more complex character who has his own needs and boundaries.
The relationships in the story are also affected. His friends might turn against him if they discover he is stealing life force. For example, Hermione would likely be horrified and try to stop him. This changes the dynamics of their friendship. Also, the wizarding world would view him as a threat rather than a savior.
The pet might introduce Harry to new magical creatures or areas. For instance, if it's a cat that loves to explore the Forbidden Forest, it could lead Harry to discover hidden places or meet other magical beings that he wouldn't have encountered otherwise. This could expand his knowledge of the wizarding world and his own powers.
In many fanfictions, Harry's view of Dumbledore could shift. After the third task, he might start to question Dumbledore's actions and motives more. He might wonder why Dumbledore didn't protect him better or why he seems to be keeping so many secrets. This could lead to some tension between them. At the same time, Harry's relationships with the other houses could improve. Gryffindor already admires him, but now Ravenclaw might seek his knowledge about the Dark Arts, and Hufflepuff could respect his courage. This could make him more of a unifying figure at Hogwarts, despite the Slytherin opposition.
The pet bat could make Harry's life more interesting. It could be his companion during lonely times, like when he's in his dormitory and the others are asleep.