 If you liked
D.Gray-man
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 ...then you might like
Enen no Shouboutai
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Young soldiers in a collective exorcising possessed people, and the mysterious villainous freaks against them.
Fire Force and DGM are very similar for their religious themes, apocalyptic threats, darker mood and gothic outlook (although the former is generally more colorful and comedic).
Shinra and Allen are both powerful orphans with a special ability, shunned since early age despite their good nature because of a creepy feature, Arthur/Kanda is the sword-wielding rival, while Tamaki is a newer Lenalee with less clothing.
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Recommended by RenaPsychoKiller
Finding the similarity in the plot, where both protagonists share a similar childhood trauma, one fights the demons, and other as firefighter fights people turned to fire demons
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Recommended by Billy_Babic
Both of these are enjoyable battle shounen with great main characters who happen to share similar backstories and circumstances and are part of organisations that put former humans turned monsters to rest. They also feature extraordinarily mysterious villains and the more you delve into spoiler territory, the more additional resemblances you find.