A concept: if the videos on Xingchen's Bili/YouTube are anything to go by, she was the first of the Medium5 sisters to materialize after ZERO split herself into five. She then travels to Earth and delivers the icosahedron to the ocean, which brings Haiyi to life; going by this pattern, we might assume Xingchen's the one who "awakened" her sisters. She's depicted as a being of light and energy, both because she literally holds power over the element of aether and because, well, she's a nice girl.
So Minus is what could have happened, had ZERO's split not gone so well. Perhaps, instead of sorrow, ZERO was filled with rage. Perhaps Xingchen herself was simply evil, or overwhelmed with the power of her element. Either way, she was born as Minus, who seeks to keep Metranon's Cube to herself to exploit its power; rather than help her sisters, she seeks to hunt and kill them, so that no one will stand in her way.
...so yeah, basically the "Minus is Edgy Xingchen" take, just slightly more fleshed out and fueled by exhaustion and caffeine.
