Shiko Devil UPDATED BACKSTORY (important new developments)
During the Demon Sword arc (starting around episode 7 of the anime, chapter 4 of the manga), there is another demon weapon that poses the threat of becoming a kishin. This corrupted soul known as the Demon Executioner is both weapon and meister in one.
The Demon Swordsman (Crona) and Demon Executioner (Shiko) are assumed to be associated, but have never actually worked together. They were both allied with Medusa around this time, but never participated in missions together. The Demon Swordsman consumed more souls overall than the Demon Executioner.
There were rumors of their existence during the Demon Sword arc, but only after they bring Crona to their side in the Trial Class arc will the existence of the Demon Executioner be confirmed. Once Crona gives up hunting human souls, souls will continue disappearing, confirming the existence of another demon weapon.
When Shiko appears at the school shortly after the Trial Class arc, there is little suspicion because she has only one soul (contrary to Crona/Ragnarok being individual souls forced to share a body) and had gained an ability similar to Soul Protect that makes her soul appear to be pure. Some meisters, like Maka, can sense that something is different about her soul. While she is attending the school, there is STILL a risk of her becoming a kishin.
Shiko acts normal enough around other DWMA students, but since her soul wavelength is very unstable she sometimes reverts back to the Demon Executioner unintentionally. This is characterized by sudden aggression, anger, and excessive violence, and she may also express the desire to consume or destroy other students' souls.
The transition to the Demon Executioner is often triggered by stress, or sometimes the influence of other peoples' wavelengths. She always instantly reverts back to the Demon Executioner when a madness wavelength is present.
The Demon Executioner's personality is similar to the Demon Swordsman/Crona at the beginning of the Demon Sword arc. She's afraid and resorts to violence to take care of things she doesn't like/understand. She doesn't value her own life and attacks recklessly. She acts a bit insane (laughing inappropriately, making nonsensical comments, stretching random vowel sounds (in Japanese, turning "ne" into "nee", "ki" into "kii", etc, causing her to pronounce "kishin" like "kii-shi-n" or "ki-shii-n") attacking without regard for her own life).
Crona IS aware of the Demon Executioner's existence, but not their identity. At most, Shiko's wavelength will feel vaguely familiar to them, but they won't make the connection themselves.
Maka will eventually recognize that Shiko's soul is unstable and appears to be partway into becoming a kishin.
Death/Shinigami-sama may forgive what Shiko has done, only on the conditions that she permanently gives up hunting human souls and allows other meisters to help her manage her anger/impulses.