Have you checked Phantom - Requiem for the Phantom ? It was the first thing I thought about when you mentioned assassin+romance...

It's dark, the romance is decent, and has good action scenes too. Only one chapter has been translated, so if you can't read Japanese you can also check its anime adaptation which is complete and was quite good.

Silver (FUJITA Kazuko) - Lots of romance and smut. Young privileged girl is tricked by her cousin. He kills her father and ships her off to boarding school where she is isolated and depressed. She finally realizes what she needs to do and works to change herself to seek revenge. Along the way she finds an ally with his own revenge. Not a heavy "action," but its got a whole "plotting revenge" thing. The real story doesn't start until she realizes what a fool she has been.

Saver - Not a heavy romance (no smut), but this is one of my favorites. The main character is very strong and is never reliant on the men around her. The scans for the first few chapters are not the best and the story really starts after she goes to the other world (chapter 4 I think?). Even still, I liked it better then "Red River" or "Basara" because of her strength.



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