Juniper is Evelyn's tame little gray mouse that resides In a roomy little pouch at her belt, or, at times, on her shoulder. Evelyn nursed Juniper back to health after finding her with a broken leg in the city streets.
While the group was at the Wizard of Wines, they used Juniper to track down Evelyn, who had turned into a werewolf and possibly ate a baby.
Juniper and the other pets got separated from the Wafflecrew in Ironslag. She got stuck inside the Vonindod and almost died. Diath rescued her from the wreckage.
Juniper died when Evelyn was slain in Barovia, but she was resurrected 50 years later by the revenants of Argynvostholt and the Vistani.
Juniper was swallowed by Summerwise the pseudodragon, but she was regurgitated before she died.
Strix kept Juniper safe while Evelyn died, and when she was handed back to Evelyn, she was covered in soot from Strix's filthy robes. 
While living in Waterdeep, the group discovered that Simon had been keeping a local boy in a box. Evelyn comforted the boy once he was freed. She let him keep Juniper as his pet.