Isha is the Eldar Goddess of healing, fertility, and the harvest.
Official Canon Edit
Isha is considered the mother of the Eldar, and has a close bond with her children, speaking to them through her spirit stones. After the War in Heaven, when one of the other gods predicted that the Eldar would kill Khaine, he decided that he would kill every Eldar to keep the prophecy from coming true. Asuryan stayed his hand by requiring all Eldar gods to remain separate from the Eldar. Isha however stayed in contact with them in secret; when Khaine discovered this, he attempted to claim Isha as his slave until Vaul intervened.
Later, when Slaanesh was born, the new Chaos God consumed all of the Eldar Gods save for Cegorach, Khaine, and Isha. Isha cried out, which was heard by Nurgle. Nurgle intervened and claimed Isha for his own, and she has since been trapped in his realm. He forces Isha to try his new diseases, which she is able to heal from.
TTS Canon Edit
What has been seen briefly is that Isha seems to have to a degree adjusted to life with Nurgle, referring to him as "dear" and defending him when Tzeentch sent pink horrors to attack Nurgles Garden. She seems to posses a snarky side passively aggressively commenting on the fact when Tzeentch laughed it was just a part of his "big old plan." She possess a clear dislike for Slaanesh due to their history and fiercely tries to have Nurgle send all the diseases at her, calling her a "infuriating penetration addict." In fact it can be said she possess an intense passive aggressive dislike for most of the other Chaos Gods save Nurgle, who, despite being kidnapping her, she shows clear affection for.