Koi Koi no Mi

Name: Koi Koi no Mi (Dense Dense Fruit)
Type: Paramecia
Abilities: Koi Koi no Mi is a paramecia type devil fruit that grants the user the ability to increase the density of organic and non-organic matter he is in contact with, without feeling the increase of the density for himself. This means that f.e. the user can increase the density of a wooden staff to be as dense as an iron staff but still be able to carry it like a wooden staff. The increase in density is not felt by the user, while it is only felt by the others. As a result, the user can increase his density to become stronger and more endurant, without losing any speed at all. When the user increases the density of a matter, there is a visual effect. The color of that matter (whether it is a living or a non-living) becomes white and shines to indicate that a Devil Fruit ability is being used.
As for the fruit's mastery, it goes like this: When the user eats the fruit he can change the density of an object up to a ROCK. After 4 threads, the user is able to change the density of a matter up to that of STEEL. After 6 threads, the user is able to change the density up to the level of a DIAMOND. After 10 threads, the user is able to change the level to that of HARDER THAN DIAMOND which is on a level between diamond and sea-stone.

Well, tell me, wouldn't this be a little overpowered then? If you use it on yourself and become as hard as diamond, but you dont feel it... without any drawbacks? Tell me, wouldn't people who use physical attacks be completely defenseless against you?

Also, how does the DF weakness to seastone and water apply to this fruit? Will things affected by the fruit lose their increase or decrease in density if touched by seastone or submerged in sea water?

I did not mention the weakness on sea-stone and water because it is the standard fruit weakness. When submerged in water the fruit power is negated from them until they are no longer in water.

As for the other part, I can make it so the user has to slowly increase the density like this

-From Normal to Rock from rock to Steel from Steel to Diamond and from Diamond to Harder than Diamond. To ascend from one state to another it will take one post so the user has to take a while to go to the maximum form and the user will have to take part in at least 5 threads to unlock a second part of the ability that allows him to instantly increase/decrease to whatever level he wants. How does that sound?

I agree that it should increase density by levels. But I think that maybe the ability of the user to increase density should take some time to master, maybe? So maybe at first you can only heighten density to the point of rock, then after [insert number of threads here] you can heighten density to the the point of iron, then to steel... etc.

Cause well... diamond really... negates most physical power towards you, as I mentioned earlier. So it should have some drawbacks.

If I make it so that a new density level is acquired when the user takes part in a few threads, can I have it so that if the user acquires f.e. the ability to increase to steel levels to do it immediately?

I guess you can, since that's what fruit mastery is about Don't you agree?

I agree. Shall I make the edits?

You should

made the edits