Hmm the story is about a little weak boy(around 12-14 years old) , he had big sister (a nun)

basically the little boy been bullied as weak by other child and being helped by his childhood friend ( a girl)

in the end , there's some sort like dunno , all people gathered in one big pit in a valley(subjugating monsters i guess), fighting many monsters and the little girl parcipate in it because she want become strong heroes.

basically the little boy come to help when the little girl nearly slashed by a big monster, the little boy suddenly got? a sword and swing it, bam killing the big boss monster instantly

and then the first chapter ended, btw it seems the little boy had blood line of legendary heroes because it seems the big sister worried about him and say " fate huh"

it had been 6 years and i want to re-read it again

can someone tell me what it is ?

Sorry if if it's not but is it perhaps Rave Master?