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Juna Fruit is an item in Fire Emblem: The Sacred Stones that lowers the level of a character (without decreasing stats), this allows a unit to increase their stats more by continuing to level up. The number of levels lowered can vary from 1 to 5. Only characters who are Level 10 or above can use it. The Juna Fruit cannot be obtained in the English version, outside of using codes or hacking. It was available from a limited-time promotional event in Japan.
In Fire Emblem Awakening, second seals can be used in a similar fashion to this, as a Second Seal can be used to reset a units level without lowering their stats.
Info
The Help display window reads, "Give this to a worthy unit and see what happens."
The item description reads, "Use this, and..."
If attempting to use while under-level, the text will display: "Must be above level 10."