Buffy ends up in Mass Effect Universe/Timeframe 40 years before Humans make first contact with council races. Appears severely injured, missing hands and feet heavily cut up along torso. A Quarian finds her, adopts her and creates hands and feet for her, however as they have never seen a human before and she looks similar to a Quarian they pattern them after a Quarian's hands and feet, give her the same implants that Quarians get.
Because of the Slayer essence's fortification of her body and the healing, she can eat Quarian food with no adverse side-effects. She ends up learning the culture and is treated like any other Quarian. She takes the name Rhea`Zorah. Has a Half-Human Half-Quarian son Rael`Zorah.
She stops physically aging at what appears 25 years. However with her being in an environmental suit all the time it isn't noticeable. Shortly after her son is born there is an accident with one of the Mass Effect Cores and she is exposed to massive amounts of Element Zero. Due to being the Slayer she survives and gains extremely powerful biotic abilities without the need of the Biotic-Amps. Her Biotic abilities are at a similar level as the more powerful Asari.
Shortly after her son goes on his Pilgrimage she leaves the Migrant Fleet as well. As she wasn't born a Quarian she never went on a Pilgrimage, she decided to go on a journey that was similar. However as she travels her Protector nature resurfaces and she helps as many people as she can. She trains her Biotic abilities with the Asari on Illum. Eventually she gains the attention of the Citadel Council. They have a Spectre shadow her and eventually they approach her about being a Spectre herself. When it is explained what being a Spectre is about she agrees.
Everyone believes she is a Quarian so is called "The Only Quarian Spectre, and the Only Quarian Biotic." The Quarians consider her a Quarian and as she constantly travels and helps the Migrant Fleet they call her the "Eternal Pilgrim". She becomes a Spectre 10 years before Humans make contact with the other races.
Everyone believes her Longevity is because of her implants and the Element Zero accident. She has extremely powerful Biotic abilities, her technical skills are average among the Quarians, and being a Slayer her combat skills are at an extremely advanced level.