Ghost in the Cell!

Spooky happenings take place in the Hinata household, and Natsumi is first to point fingers at Keroro - but Keroro's air-tight alibi leaves everyone puzzled. This somehow leads to bigger unexplained manifestations, and finding the logical explanations for them leads to one final manifestation, as the Hinata house ghost is sick and tired of everyone forgetting her existence by simply explaining it away.
A ghost with a grievance is a powerful thing indeed, as the house ghost spirits Natsumi and Keroro away, and her long and painful history is revealed at last - as a child, she found a kappa (an amphibious Japanese demon), befriended it, and was deemed a heretic and locked in a vault by her fellow villagers for it...