original description

Nine still has the other tails! Just more advanced. They are like projections, except they're very much real, they can and will hurt you! (glances at eggman)

Some TTAU info,

Nine still has the other tails! Just more advanced. They are like projections, except they're very much real, they can and will hurt you! (glances at eggman)

Over time, it's just easier to not carry around several extra metal limbs, right?

The metal tails were developed in the normal "green hills" world. When Nine was first transported through the shatterspaces, it was only in the blueprint phase, they later built it along with some input from Tails (which wasn't well received by Nine, since they were very distrustful and wary at the time of arrival..)

Classic Nine walked around with the metal tails for a fairly long time before changing it to it's more practical version, Tails did help again, but at that point they already were closer :)