If you liked
Shi ga Futari wo Wakatsu made
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...then you might like
Marginal Operation
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Kids stuck in the middle of conflict, protected by adults who they protect in turn. Marginal operation is darker and more military, while Til Death Do Us Part is more of a shounen battle manga. They each have strong action and characterisation. Note that Marginal Operation is NSFW while Til Death Do Us Part is not.
If you liked
Yokohama Kaidashi Kikou
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...then you might like
Shoujo Shuumatsu Ryokou
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They both have a very similar, "Cosy Dystopia" feeling. Girl's Last Tour is certainly darker, with YKK presenting a more idyllic take on humanity's end.
If you liked
Hitsugikatsugi no Kuro.: Kaichu Tabi no Wa
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...then you might like
Majo no Shinzou
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This follows a similar idea, a strange young women travels the world on a quest followed by strange companions, a talking lamp in "Majo no Shinzou" and a talking bat and odd children in "Hitsugikatsugi no Kuro". Both of them follow an episodic structure and have some fairy-tale elements. "Shoulder-a-Coffin Kuro" is more consistently dark in terms of tone whereas "Witch's Heart" flip-flops between light and heavy themes more often. That's not to say that either can't manage emotional scenes or comic relief either.