Radiance by Alyson Noel

[that's Riley and Buttercup]

From the #1 New York Times bestselling author of the Immortals comes a new middle-grade series featuring Ever’s younger sister, Riley.
Welcome to the Here and Now
Riley Bloom left her sister, Ever, in the world of the living and crossed the bridge into the afterlife—a place called Here, where time is always Now. Riley and her dog, Buttercup, have been reunited with her parents and are just settling into a nice, relaxing death when she’s summoned before The Council. They let her in on a secret—the afterlife isn’t just an eternity of leisure; Riley has to work. She’s been assigned a job, Soul Catcher, and a teacher, Bodhi, a curious boy she can’t quite figure out.
Riley, Bodhi, and Buttercup return to earth for her first assignment, a Radiant Boy who’s been haunting a castle in England for centuries. Many Soul Catchers have tried to get him to cross the bridge and failed. But he’s never met Riley. . . .