Luffy's acquired plenty of powerful forms which have come in all sorts of weird shapes and sizes, some looking much better than others.

Of all the transformations and power-ups in One Piece, none come close to the level of importance as Luffy's Gear 5. This form not only transforms him into the legendary warrior of liberation ...

Both these versions of Nika seem to be currently personified by Loki and Luffy respectively, with Luffy representing freedom and liberation while Loki represents chaos and destructiveness.