If you liked Hunter X Hunter, you should watch Yu Yu Hakusho since they were both created by Yoshiyuki Togashi. It's about a delinquent named Yusuke Urameshi who after dying and being brought back to life, gets asked to become a Spirit Detective, which is honestly just a fancy title and he's more or less the protector of the living world, it pretty much meets all of your requirements.
Little Witch Academia is perfectly appropriate for your age group as it's basically the anime equivalent of Harry Potter, both Little Witch Academia films are available on Netflix.
Full Metal Alchemist is a anime series about Edward and Alphonse Elric who after attempting the forbidden act of human transmutation, end up paying a terrible price; Ed loses both his arm and leg and Al loses his entire body. Devastated by what they're done, they set out on a journey to hopefully find a way to return to their original bodies. Watching Full Metal Alchemist can be somewhat confusing before you even start, because there are two versions, 2003 and Brotherhood. FMA 03 is known for only adapting the manga's source material up to a certain point when they ran out of material to adapt and then with the manga author's blessing, continued on with their own original story. Brotherhood is a more recent and more faithful adaptation to the manga, however I don't believe that makes 2003 absolute by any means, in fact it's the version I prefer. I honestly suggest recommend watching 2003 before Brotherhood even though Brotherhood does cater more to your criteria, the fact remains that most people find it more difficult to watch the older brother to FMA after watching Brotherhood not only for its age, but for it's difference in pacing and tone but I ultimately feel that watching it is still a experience worth having. Both versions of FMA are available on Netflix and Hulu.
