The changes had come quickly, and painfully. He had had to put permanent silencing charms around his bed so no one would hear him. The first and most painful change was the tattoo. That was proving fun to hide from everyone. The main portion of it covered his entire back with the symbol of the Hallows. Well, when he said the Hallows were tattooed, really that was what wasn't inked onto him. The symbol formed from the skin that was showing through the swirling and ever moving mess that appeared to be the souls of the damned. It was mostly on his back, but at times, the blackness would elegantly seep and swirl across the rest of him. It was quite pretty actually.
When questioned, Death simply shrugged and stated plainly:
"I thought it looked cooler." Harry supposed that meant it wasn't the souls of the damned after all. He wasn't sure whether he was relieved or disappointed.
Next came his height. Now, one would expect him to rise up above his decent height of 5'9", but he was Harry Potter, and things didn't work that way when it came to him. Instead he had shrunk down to an underwhelming 5'3".
Along with this came a shift in his bone structure. He had taken on a more effeminate, androgynous look, similar to Death's own. They definitely looked related now. These changes were gradual, and took about a week to fully settle, and made him extremely sore during that time and for a couple days after. He also found that his eyesight was fixed. He enjoyed destroying the glasses.
He noted happily that he could still speak with snakes. Although everyone else had despised the power in second year, he had secretly loved it. The garden snakes had been his only conversation while at the Dursley's. Even during the trio's impromptu camping trip, he had continued to talk with the serpents of the forest, to get away from the stress of it all for even just a few minutes, to talk about something other than Voldemort and their impending doom if they didn't do something right now Harry.
The magical power boost Death had mentioned arrived, and he was now able to perform simple things with more of a thought and a wave of his hand than an actual spell. However, he did continue to use his holly wand, so his friends wouldn't start questioning why he was suddenly proficient in both non verbal and wandless magic.
He wasn't planning on ever telling them about the whole Master of Death thing. Harry knew how'd they, and everyone else would react, not to mention the stories the Prophet and the Ministry would spread about him, never mind the fact that he had just saved the entire wizarding world from the Dark Lord.