(The) Jar of Pickles looks like a normal jar of pickles, but it is actually a Deventi, a shape shifting monster. Madge Griffin gave it to Diana Caldur as a gift to start her journey with her fellow crew. Kaori West is constantly trying to open it, but it has been magically charmed that only Diana can open. The only one who knows of this beast is Kai and Diana herself.
It can only be opened and controlled once, before returning back to sea. In order to catch another one, you must first gain it's trust before asking it to take the form. (Ironically, Madge asked for the Deventi to turn into a box, and it changed into a jar of pickles.)