Very little is actually known about the Yama's background. According to statements by her subordinate, she was once a jizou statue who had received enough faith to gain divine powers, and was promoted to her Yama status. Ever since then, she had been assigned to be the Yama of Gensokyo. She resides inside of Hell, sometimes Higan, and judges the dead that come before her. It is her job to determine if they are to be sent to Hell, Heaven, go to Hakugyokurou, or get Reincarnated. She also has no discrimination between youkai and humans. (Evidenced as she told Yuuka to scare humans more, and Reimu to exterminate more youkai.)
Despite her reputation as being an annoying woman that nags based on someone's flaws, she rather approaches her job professionally, keeping her scolding aside when she's on her days off, going around Gensokyo to observe and give advice. Though it is her job to judge people based on their actions in their life, she isn't against warning people to change their ways before she is forced to give them the sentence to hell. Only then does she release her pent up rage and anger at them before her stand, and pass judgement. 'DANZAI'
Crystal Mirror: What looks like a normal hand mirror is actually an important device used by the Yama. This mirror allows Shikieiki to see into someone's past, and look into their past events. This is useful in judging those that are unwilling to talk to her, or to check on the status of one that she's recently given advice to. The mirror is very powerful, however, it also drains Shikieiki of a lot of energy, and is only used sparingly. It can be used to find out just about anything, and is a blatant invasion of privacy, but Shikieiki only finds the information she needs from the mirror.
Power Level: VEGETA! WHAT DOES THE SCOUTER SAY ABOUT HER POWER LEVEL? To be honest, Shikieiki's power has never been displayed, but the gap youkai herself has stated that even she, Reimu, and Yuyuko teamed up would still be no match for the Yama. (ZUN's 'A Flower Blooming Fragrant Violet Every Sixty Years' is source.)
Spellcards: Although Shikieiki rarely gets into danmaku fights, her very few cards are actually considered extremely dangerous. She had very few cards before, but she's gone to create a few more for herself in spare time, perhaps to help persuade sinners to change their ways. Many of her spellcards are named after her occupation and the Yama, though none of her spell card patterns relate to the name... except for the one that utilizes her Crystal Mirror.
Streetwise: A friend to honest people, and an annoyance to those with low karma levels. She is known to visit those with many sins, and warn them of their fate after death and urge them to change their ways.
Komachi Onozuka ~ Eiki's assistant, Komachi, is a good friend and worker... when she's not sleeping on the job. Though the judge has to visit her and make sure she's doing a job, she usually cuts deals with the shinigami, promising something rewarding if she isn't spotting sleeping when she checks up on her.
Ishimaru Kaito ~ Eversince his first impression to the Yama, Eiki has hated Ishimaru. The fact that he openly threatened a Yama has really been on his nerves. Though he may be working his way back into the white of things, Shiki Eiki is keeping a close eye on his mess ups, ready to scold him if need be.