The Crimson Lightning Demon (Male reader insert x Danmachi)

"What is your dream young ones?" Asks an old man to a pair of children in his lap,one a white haired red eyed girl named Bella the second a dark red haired black eyed boy named (Y/N).
"Too be a hero!!" They both said in unity getting a chuckle from the old man
"That's a fine dream, but promise me one thing you two" he said in a serious tone
"What is it grandpa?" Asks Bella
"That you'll always look out for each other no matter what" he says
"I would have done that regardless if you asked, no one will hurt my baby sister" (Y/N) said getting a blush from Bella
"I know you will my boy, you do have my magic after all" he says before me and Bella fall asleep on his lap

That promise is the reason I live, to protect my sister and those I love from all harm weather it's a God or monster they won't touch them so long as I breath

Here's the idea of the story, what kind of magic do you have?, will your sister be the only one you protect?, will you find your and Belle's parents?, read to find out.