 If you liked
Saint Seiya: The Lost Canvas - Meiou Shinwa
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 ...then you might like
Kimetsu no Yaiba
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Both manga have a main character who has a relative that fell into evil (though both approaches are different) and the plot deals with them trying to save that sibling figure from their darkness accompanied by two friends he meets during his journey. As the series goes on, the focus starts to be given to the elite members of the main benevolent organizations of the setting, with them receiving spotlight and being fleshed out through the many arcs of the manga.