Act 1 - Blighted Village
In the Apothecary's Cellar, there are a number of coffins. Inside one of these is a "Scroll of Summon Quasit." When you use this scroll, Shovel appears as a one-time minion/summon creature.
If you have a wizard in your party, it is possible to teach them the spell, allowing you to resummon Shovel at later points. There is also a way for Sorcerers and Warlocks to permanently gain this spell after summoning Shovel, but it involves a missable dialogue option when speaking with her.