 If you liked
Tenkuu no Shiro Laputa
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 ...then you might like
Green Legend Ran
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They are both stories with an idealistic and capable male lead (Pazu/Ran) that comes to the aid of a mysterious female lead (Sheeta/Aira) that is connected with otherworldly powers or entities. Laputa and The Holy Green are both unknown and forbidden landmarks that offer entryway into an unknown power that has the power to destroy humanity (or most of it). There are rival factions that oppose one another as well (Colonel Muska/Rodoists vs. Dola Gang/The Hazard) and the two leads end up involved as well.