The fourth doll of the Rozen Maiden collection and Suiseiseki's twin, she possesses a sharpness in her speech that is similar to Shinku's, attacks ruthlessly in combat, and does not tolerate sub-par actions. Her appearance is not only the most masculine of the dolls, but her speech is also noted to be of the masculine form, using the first person pronoun "boku" instead of gender neutral pronouns like "jibun" or "watashi." Souseiseki's presence is much more significant in the anime than in the manga.
In the manga, Souseiseki's master wanted to use her power to enter other people's dreams and exact revenge on his brother's lover, who he felt had stolen his brother from him. Since Souseiseki alone could not achieve this goal, she fights Suiseiseki to gather the two gardeners' tools, the shears (which is her primary weapon) and the watering can, which together can affect an individual's soul tree. She eventually realizes that it is her master's heart that needs to be freed and sacrifices herself to do so. The dolls present are shocked by Souseiseki's actions and Suigintou uses the confusion to snatch Souseiseki's Rosa Mystica, enraging both Shinku and Suiseseki.
In the anime, Souseiseki is devoted to a lonesome old man, who uses her as a substitute for his deceased son, Kazuki. She is shown to be slightly more eager, probably from a sense of duty, to participate in the Alice Game than her sisters. In the OVA, she is introduced in the flashback in mid-fight with Shinku and later does not hold back in attacking a then-harmless Suigintou. Throughout Rozen Maiden Träumend, she questions her idyllic lifestyle, and, much to Suiseiseki's disappointment, takes part in the Alice Game out of devotion to Rozen, to fulfill his wish of finding Alice and relieve his perceived grief. She battles Suigintou, but is defeated and loses her Rosa Mystica.
Even though the two versions progress differently, Souseiseki is still the first doll to lose her Rosa Mystica, and her Rosa Mystica transfers to Suigintou on both occasions. However, at the end of Rozen Maiden träumend, Laplace is seen dancing with two Rosa Mysticae, one of which is believed to be Souseiseki's. To add to the confusion, after Shinku is resurrected by "Father," she tells Jun that Souseiseki's and Hinaichigo's losing their Rosa Mysticae is her burden to bear. The most likely reason for this is that essentially they were under the care of her medium, which makes sense for Hinaichigo, but not so much for Souseiseki. Recently in the manga, Souseseki is shown to be missing from her box. Though she is the first doll to lose her Rosa Mystica, Souseiseki is actually the second doll to lose in the Alice Game. Her artificial spirit is named for a fragrance by Lolita Lempicka. Souseiseki was also the first runner-up for Saimoe 2005.