In Inuyasha who is Rin?

Answer 1

A) Rin is a human girl who travels with Sesshomaru (Inuyasha's older, full-demon brother) and Jaken, Sesshomaru's servant.

B) Rin is a human orphan girl who originally lived in the village that was attacked by Koga's wolf pack when he first appeared in the series/manga. Sesshomaru had been injured (It might have been when he was fighting InuYasha (when he lost his arm) but it probably wasn't.) and he was resting in the woods waiting for his body to heal, when Rin had found him, noticed that he was injured, and even with his protests had continued to bring him food and water.

When Sesshomaru's body had finished healing, he was walking on a trail leaving the village with Jaken by his side, when he came across Rin's dead body; she had been attacked by some of Koga's wolves. Sesshomaru was going to just ignore her and go on his way, but he was stopped when Tenseiga had pulsed, implying that he revive her (he took it as a test to see what Tenseiga could do). When she had come back to life, she had realized that Sesshomaru had saved her, and for some reason I don't know of, decided to follow Sesshomaru.

She has been traveling with him and Jaken ever since.