A Hirami Lemon is an extremely bitter citrus. The fruit grows from a giant tree in icy Helman and can also be found underground in the citrus plantation in JAPAN. The fruit has a special property, it has the ability to delay the transformation of an individual into the Demon King. An example of this can be seen with Miki, who has managed to maintain mostly human for the last 5 years, ever since Gi gave her the power of the Demon King. At one point, Miki and her boyfriend Kentarou managed to escape the continent and returned back to their own world, but they eventually ran out of Hirami Lemons and had to return to the continent because she would have gone to slaughter the people of her world. Withdrawal from the lemons will have very rapid effects on the host, as seen at one point when Miki lost all of her collected lemons in a fire in JAPAN. After only a couple of day, during a stressful encounter, Miki temporarily completed her transformation into the Demon King Little Princess and it was only with the consumption of more lemons that she managed to revert back to Miki.
As stated before, the Hirami Lemons are only a temporary measure, and eventually a combination of force of will and the lemons won't be sufficient to suppress the Demon King transformation. The purpose of the lemons on the continent has never been explored, such as if they exist for a Hero to apply the juice of the lemon on their blade in order to weaken the Demon King in their final showdown.