Sakuya is a curiosity to many of the people of Gensokyo, very little is actually known about her. Though she has managed to keep much of her past quiet she has expressed on several occasions the reason for their move to gensokyo had something to do with some form of problem or danger occurring where the mansion was previously located. She has expressed several times her respect for Gensokyo and its peoples and has implied a lack of it for her own past. Both this and the vicious nature of her non-danmaku combat style has led people to speculate on the exact nature of this previous world.
Sakuya is a human who acts like a youkai. She acts with basic respect toward people she doesn't know, and, for some reason, is less kind toward other humans. After knowing someone for a while, like Reimu or Marisa, she will gradually warm up to them. To anyone who threatens her home or the residents of the SDM, however, she is ruthless and frighteningly violent. She also has a very high work ethic, never slacking off and only taking a break when work is done.
Voice Sample: http://www.youtube.com/watch?v=bPMpT3hzWV4
Remilia Scarlet ~ The person Sakuya has the greatest devotion to in the world would be her mistress, Remilia Scarlet. She would do quite literally anything in service of the little vampire, and has the utmost respect for her. On occasion, however, Sakuya can't help but act like a surrogate mother to Remilia.
Flandre Scarlet ~ Sakuya has a similar devotion to Remilia's younger sister, Flandre. Sakuya acts more motherly to the young mistress than she does with Remilia, primarily because the maid pities her. Though she fears Flandre's potential power, Sakuya still does all in her power to make the Flandre feel comfortable and stable.
Hong Meiling ~ Sakuya's best friend and co-worker, Meiling is the person Sakuya will confide in and generally be more relaxed with. The maid often falls victim to Meiling's teasing due to her more serious and rigid personality, but they both know it to be in good fun. In return, Sakuya often berates Meiling for slacking on the job, but the gatekeeper never seems to change.
Patchouli Knowledge ~ Sakuya is on good terms with Patchouli. Despite the cold professionalism she is quite fond of the magician and often holds her in a similar regard to Remilia.
Reimu Hakurei ~ Sakuya is on good terms with the Hakurei shrine maiden. The two have solved several Incidents together, and the maid has gained a significant respect for her.
Marisa Kirisame ~ Sakuya greatly dislikes the resident witch of Gensokyo. Every time she breaks into the mansion, several books are stolen, the building is often damaged, and the general dignity of the Scarlet Devil Mansion is dislodged. Not to mention the increased scorn the maid gets from Patchouli after every invasion.