Yeena is an NPC that can be found in The Forest Encampment in Act 2 and in The Bridge encampment in Act 7. She functions as a vendor of magic/caster items as well as flasks, and has a significant role in several quests.
She is a very kind-hearted and somewhat eccentric person, appearing even slightly child-like in her personality. She firmly believes that she can communicate with nature and "the spirits", to which some residents of the Encampment react with scepticism or amusement, but it turns out that at least sometimes, her visions are not out of touch with reality.
Yeena is a wild soul in touch with nature. She seems to be able to foresee things and offers a unique perspective on the world to the exiles who pass through the Encampment. She appears to be very compassionate and willingly helps anyone in need - for example, she takes care of Helena when she appears in the Encampment, even though it is not initially certain whether she can be trusted.
Silk appreciates Yeena's unusual character and that she focuses on the more spiritual side of life, rather than only on the mundane, physical reality. Some others are more sceptical, however - Greust seems convinced there is no truth to her visions and that thinking she talks to the spirits, "she talks to herself", whereas Eramir also appears quite sceptical and worried she may be meddling with powers she should avoid. In the end he does admit, however, that her more vivid visions often prove true and can be very helpful (as is the case in the final quest, Shadow of the Vaal).
You arrive! The great Spirit spoke in my dreams, told me that darkness would again drown our lands. And you, exile, would walk before the flood.
But what is this? You are not alone. A shade a memory, older even than...no...older than Spirit? Fear grips my throat, I shake, but the Spirit drapes a warming cloak upon my shoulders.
This ancient ghost that follows you, it has the trust of the Spirit, and if the Spirit trusts then I trust.