Important NPCs
Gensokyo is filled with characters who Yuyuko interacts with, the following are the most important/have history with Yuyuko.
Konpaku Youmu
Youmu is the gardener of Hakugyokurou in name, but she also serves as a bodyguard, sentry, and companion for Yuyuko. She is easily the most important person in Yuyuko's everyday existence, and the two are seen together a lot, save when Yuyuko tells Youmu to stay behind to watch the entrance to the Netherworld, or when Youmu is doing errands for Yuyuko. Youmu seems very fond of Yuyuko on top of being a loyal servant, and thus will defend her to no end. Youmu is the only fighter in all of Gensokyo to rely solely on her own strength and agility to win her battles. She uses twin katanas, and is a very competent fighter, having studied under her father for the better part of her childhood. Youmu is part ghost, though she lacks the ability to pass through solid objects like Yuyuko. Her ghost half hangs about her at all times, a white spirit that tends to circle her shoulders.
Yuyuko's feelings toward Youmu are unknown. It's possible she cares for Youmu, though her present state, being dead, keeps her from being able to feel such things very deeply. She does seem to quite enjoy being with Youmu, however, and mentions her often.
Yakumo Yukari
The only character who knew Yuyuko when she was alive, Yukari is Yuyuko's best friend. Even though Yuyuko has forgotten her friendship with Yukari so many years ago, she feels a connection with Yukari that keeps them close. In fact, Yukari is the very reason Yuyuko is now a ghost princess, as Yukari used her powers to manipulate the border between life and death to give Yuyuko a happy and eternal afterlife. Thus, Yuyuko will never be reincarnated, and will never be burdened with the pains of living once more. Yukari firmly refuses to speak of Yuyuko's past, as she doesn't want to bring back painful memories. Yukari is a nocturnal demon, and has the ability to manipulate borders, a power that is almost unfairly potent. She is often placed on the same level as gods in terms of power-- it's a good thing she's relatively harmless. Yukari seems to love Gensokyo with a passion that rivals all others, and always returns despite her ability to leave for the outside world at any time. She has a habit of using her border manipulation to pop up at odd times, often being in two places at once. And yes, she can grope you from across the room.
Yuyuko is drawn to Yukari, and trusts her immensely. The relationship between the two of them is a bit beyond friendly, but due to their natures as a carefree ghost and demon, couldn't exactly be called romantic.
Hakurei Reimu
Reimu is the priestess of the Hakurei Shrine in Gensokyo, and one of the three humans to cross into the netherworld, breaking the boundary between the living and the dead. Reimu once fought Yuyuko in order to return spring to Gensokyo, as Yuyuko had instructed Youmu to steal it. As Yuyuko was never able to revive the demon cherry tree due to Reimu's intervention, Yuyuko would owe her non-life to Reimu... If Yuyuko had any idea about the concept of being in debt. Instead, Yuyuko tends to pester Reimu at the shrine on a regular basis, usually for food or tea. Reimu's battle tactics lie in her holy powers, she uses ofuda and holy magic to battle, and is quite fearsome when she gets serious.
Kirisame Marisa
Another one of the humans who crossed into the land of the dead, Marisa is mostly known to Yuyuko through Reimu. Marisa is cheeky and obnoxious, but manages to get along eventually with everyone she meets, Yuyuko included. Often times Yuyuko will join Marisa in finishing off a bottle of sake at shrine festivals. Marisa is a magician in practice (though a human by birth) and is a very strong magic user. Her powers lie in black magic, and her most powerful move is "Master Spark," which is a very explosive and destructive spell.
Izayoi Sakuya
The third "human" to cross into the Netherworld, Sakuya is the head maid at the Scarlet Mansion. Yuyuko is typically rather uninterested in Sakuya, as all she tends to talk about is cleaning. Now and then, though, Yuyuko will challenge her for some "enlightenment" and the resulting battle is usually impressive. Sakuya has the ability to manipulate time, and throws multiple knives (only to stop time and add more, a la Dio Brando) thus making her a challenging opponent. It is unknown whether she's really human, and Yuyuko really doesn't seem to care if she is or not.