Gyuu Gyuu no Mi

The Gyuu Gyuu no Mi (Kanji: ぎゅうぎゅうの実, Figurative English: Whimper Whimper Fruit) is a Paramecia-type Devil Fruit which permanently causes the physicality of the consumer to be affected by his or her hope-despair ratio.

Advantages and Disadvantage
The Gyuu Gyuu no Mi causes the physical capabilities of its consumer to partially determine by his or her mentality in a more direct manner than usual. A hopeful consumer will be stronger, faster, more flexible and et cetera. While a despairing consumer will be weaker, slower, stiffer, and et cetera.

The consumer is still required to work for superhuman capabilities, but the effects of this fruit can provide a boost that can take human capabilities to superhuman and superhuman capabilities to hyperhuman or whatever you call the step beyond.

Where There Is Life, There Is Hope
Where There Is Life, There is Hope (Romaji: Inochiattenomonodane, Kanji: 命有っての物種) is a maneuver where the consumer derives hope from the idea that obstructions only exist to prevent you from doing what you can do. Thus anything people try to obstruct you from doing can be done. By deriving hope from this idea, the consumer can derive strength from others' attempts to obstruct their actions. Meaning the more you fight against him or her, the stronger the consumer becomes.

To Be Plunged Into Darkness
To Be Plunged Into Darkness (Romaji: Menomaegamakkuraninaru, Kanji: 目の前がまっくらになる) Is the worst case scenario of a consumer of this fruit. If the consumer's despair reaches beyond the consumer's Despair Event Horizon, the consumer's body will literally collapse into a black hole of sorts.

Those who are either stupid enough to venture into the black hole or unfortunate enough to have been drawn into it will find themselves within the very consciousness of the consumer. The only means of escape from the consumer's consciousness is to decimate it to the point where the consumer loses his or her sentience.

This will render the consumer deceased and as a result all of the lingering effects of the Gyuu Gyuu no Mi will be immediately banished into non-existence.

Trivia

 * Gyuu is a Japanese onomatopoeia for the sound people make when they're being criticized.
 * This fruit was named such as both a means of dissuading people from eating it and a warning about this fruit's Worst Case Scenario.