Adria

Adria the Witch returns from Diablo 1

Adria is the Witch merchant from Diablo 1. She sold and bought potions, scrolls and staves. She also recharged staves with their magical charges. She was not in Diablo 2, having fled Tristram in anticipation of the attack on the village.

She will be featured in Diablo 3 although it is not known in what capacity.

Jay Wilson mentioned Adria in an interview at Gamescom, August 2010.[1]

Jay Wilson: We’ve not come up with one yet. But we’ve brought chars back from previous games. Adria is the witch from original Diablo. She’s coming back as a fairly significant character.

The Mystic?

With the announcement of the Artisans, fans immediately began to wonder if Adria might be the [[Mystic]. [2]

She may be; one German interview called the yet-unnamed Mystic a "he," but that might just have been a transcription or hearing error. Besides, the Blacksmith and Jeweler are confirmed as males, so it would be odd if the Mystic was also; there's usually more variety/diversity in gender than that, in the Diablo games.

We won't know for certain until Blizzard reveals the truth.