YukaReimu

YukaReimu is the femslash ship between Reimu Hakurei and Yukari Yakumo in the Touhou series.

Canon
Yukari has always had a close tie to the Hakurei Shrine maidens, as one meaning of her name is she controls the Shrine Maidens of Gensokyo. Reimu, along with Marisa, first met Yukari in Perfect Cherry Blossom when they asked her to repair the border between the Netherworld and Gensokyo. From their conversation in the Phantasm Stage before battle, it is possible to infer that Yukari has actually known Reimu (and Marisa) previously.

As a friend, Yukari often visits Reimu at the Hakurei Shrine. She steals food from Reimu in Perfect Memento in Strict Sense, stays at her shrine for at least a month into the winter in Curiosities of Lotus Asia, and is among several characters when Reimu complains about youkai visitors to her shrine in Wild and Horned Hermit.

Yukari teases Reimu relentlessly and is frequently obstinate and unhelpful, but also gives Reimu important guidance, like clues to who the current incident culprit is. As a mentor, she trains Reimu to use her powers more effectively.

Yukari also occasionally seeks Reimu's assistance in resolving incidents in Imperishable Night and Subterranean Animism.

Fanon
YukaReimu is the most popular pairing for Yukari and the second most popular couple for Reimu (just behind ReiMari). There are currently 3070+ illustrations tagged with ゆかれいむ (YukaReimu) on pixiv.

Their relationship in fan works is usually identified by Yukari playfully teasing or flirting with Reimu (as Yukari does poke fun at her a lot in Imperishable Night), who often acts irritated or impatient with the advances but may become receptive to them eventually.