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In 1920s America, Sister Rosette Christopher is a young nun of the Magdala Order - a military branch of the church charged with exorcising demons. Together with her assistant and best friend Chrno, a demon who Rosette has a contract with, she is sent on various missions to fight the many different monsters that lurk in the shadows. However, Rosette's main motive is to find information regarding her brother Joshua, who was kidnapped by Chrno's former ally Aion for unknown reasons. Aion has an ambitious plan that threatens the future of the world, and it's up to Rosette and Chrno to figure out Aion's plans and rescue Joshua before time runs out...
Both of these series deal with relations between super natural creatures and the catholic church. Friendship and coexistance between species is also a big theme. People may not take Chrono Crusade seriously at first due to its comedic nature at first, but it gets serious in the later volumes.
For centuries, vampires have lurked in the shadows of England's streets. The Hellsing Organization, currently led by Integra Hellsing, has spent centuries fighting these monsters and keeping their existence a secret from the world. Alucard is a vampire sworn to serve Integra and follow Hellsing under her rule; Seras is a fledgling vampire subservient to Alucard, learning about vampire ways while struggling with her own lost mortality. Together they keep England safe from blood-drinking monsters, until a new breed of pseudo-vampire, the Freak, begins attacking people for fun and not to feed. Where are these Freaks coming from and what will Hellsing sacrifice to stop them?
Hellsing and Trinity Blood have many similar themes, though are different in their execution: Hellsing leans much more towards the shonen/seinen demographic, while Trinity Blood is arguably more of a shoujo style. Both stories, however, deal with vampires (and the monsters that hunt them), the church (though they are the protagonists in TB, and antagonists in Hellsing), and have casts of characters that parallel each other nicely.
Similar concept of post apoclytic world. Rain and Abel Nightroad are effectively the sam person, both with "immortality" and a dark past. Trinity blood is however more violent than Immortal Rain.