To explain further, I have to explain my version of Hell first. Hell in my setting is a place where souls with immense amounts of willpower can actually thrive and escape is possible through a lot of means. By having enough resolve, one can shrug off whatever Hell can throw at them and recover from damage done by demons. If one has really high amounts of determination, they could even attain a very limited version of reality-bending and gain all kinds of powers such as creating weapons out of nothing or fire some dumb laser beams or some shit. Most of the demons in Hell are actually souls with a decent amount of mental fortitude to not become some eternally tortured scrub but had been corrupted by the soul-warping nature of Hell. Every time someone gets sent to Hell, they just spawn in some random spot to eventually have their souls flayed by Hell's landscape, or its denizens. Yeah my Hell is fucking stupid. I haven't really touched upon this thing much.

Back to Menma, she's a character that explores the concept of "How far will someone go for her beloved?". She had a special someone, but whether that love is reciprocated or not is something I don't want to dwell upon yet. Anyways the dude died for some reason, and through some means Menma finds out that he's somewhere in Hell. Guess he was a bad boy. After hearing this, Menma goes on a killing spree in order to follow him through the depths of Hell, not stopping until she dies. And then she did die, and that's how it all begins. Menma has been trying to find this guy in the infinite expanse of Hell ever since. She eventually finds multiple ways to get out of Hell throughout her adventures but tbh I still haven't thought about as to why she still returns to the mortal realm and have a school life instead of focusing on his guy in Hell.

Menma was made to have an absolutely unbreakable will, and in my version of "Hell", that translates to Doom Slayer-tier powerlevels. She also had an affinity for knives I guess and can materialize knives whenever she wants. She also made this Unlimited Knife Works ability of hers also manifest in the mortal world as well because why the fuck not. She is also "immortal" in some sense, coming back even if you do manage to kill her. Though her return might take a few hours to a few weeks. I also wanted to explore the concept of an immortal character who has to work hard coming back to life and not just reconstruct their body and then wake up a few moments later, that's why I made her the way she is.

I could go on more about her character but that's basically the gist of it.

Also as to why Minori was sent to Hell, she isn't the kind and pure-hearted character that she appears to be.