Sandy is a villager who lives in the Calico Desert where she runs the store, the Oasis. Players won't meet her until they've repaired the bus by completing the vault bundle or purchasing "Bus" for 40,000g through the Joja Community Development Form.

Sandy allows Mr. Qi to operate his casino behind her shop. It's clear they're not close, but she accepts his money and "looks the other way" with whatever he's doing back there. She also mentions she's friends with Emily, as during the first meeting she mentions that Emily wrote to her about them.
When the player completes all chat options, they will sometimes be given Milk. This is limited to once per day.

Source - Stardew Valley Wiki