 If you liked
Kishin Houkou Demonbane (TV)
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 ...then you might like
Code Geass: Hangyaku no Lelouch
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Both series contain a dark theme and background.
Additionally, both series' main protagonists lives are changed forever when they meet a mysterious girl who grants them a power in the pilot episode.
Both series also elements of action, drama, comedy, mecha, and later romance.