This is a list of Riku's forms in the Kingdom Hearts series.
Kingdom Hearts Birth by Sleep version
When Riku was a child, as seen in Kingdom Hearts Birth by Sleep and flashbacks in the original Kingdom Hearts, his hair is shorter and his bangs are slightly side-swept. He wears a high-collared, blue-trimmed, sleeveless yellow shirt with a zipper that reaches halfway down to his chest. He wears black shorts and white shoes with grey soles and blue stripes on both sides. Riku also has light blue wristbands on each of his wrists, along with a thinner, yellow band around each blue one.
Kingdom Hearts version
In Kingdom Hearts, Riku's hair is slightly longer than as a child. He wears a yellow-and-black tank top with white-trimmed, black straps crisscrossing his chest and upper back with a white on each end. He sports a pair of gray-trimmed, black gloves with matching black wristbands, dark blue pants with brighter blue baggy pant legs connected with white-studded suspenders and are buckled with small black straps on his ankles. On his hip, he wears a black belt with a silver rectangular buckle. His white-and-blue shoes have white-trimmed, black straps, gray soles, and are decorated with yellow strings on the inner sides, some of which crisscross each other.
Kingdom Hearts II version
In Kingdom Hearts II, when Riku returns to his normal form, he has a sleeveless, black zip-up tank-top with double zippers, and a white-and-yellow vest over it with a blue separating the two colors and decorated with four silver buttons. He sports a white thick band on his left wrist, baggy blue pants held up by a black belt covered with square spike studs. He wears a pair of yellow-laced, white sneakers with gray lining and soles, and three black stripes on the sides. His hair is much longer, reaching down to his upper back. Riku reprises in this outfit in Dream Drop Distance, but has cut his hair to a shorter length.
Kingdom Hearts 3D: Dream Drop Distance version
In Kingdom Hearts 3D: Dream Drop Distance, Riku's appearance reverts back into that of his fifteen-year-old body when he is the Sleeping Worlds. His hair is about the same length as it was as a child. He wears a white, black-trimmed zip-up vest with yellow lining on the sides and a black high collar. The vest also has a black sigil for the Spirit Dream Eaters on the back, indicating Riku himself becoming a Dream Eater in Sora's Dream. He also wears a pair of black wristbands, white-blue pants with a black belt with a silver buckle, and white shoes with black laces and a yellow lining on the bottom just above the black soles.
Kingdom Hearts III version
Riku wears a black, hooded jacket that he keeps unzipped, with short sleeves, gold buttons, and a checkered pattern at the hem. Underneath, he wears white shirt with a V-neck. He wears blue cropped trousers that are hemmed with the same checkered pattern as the jacket, black glovelettes on each hand, and black high-top boots. His hair remains the same length as it was in Kingdom Hearts 3D: Dream Drop Distance, though it appears more spiked up in the back after he gets a haircut from the Demon Tower.
Halloween Town version
While on Castle Oblivion's Halloween Town floor, Riku's skin takes on an ashen and slightly decayed look. Unlike Sora, Donald, and Goofy's drastic costume changes, Riku's outfit remains otherwise the same as normal. These changes also apply to his Soul Eater and Dark Mode as well.
This version is exclusive to Kingdom Hearts Re:Chain of Memories.
In The Grid, Riku wears a dark blue suit with light blue circuitry that bears a resemblance to his outfit from Kingdom Hearts, and a visor characteristic of participants in the Grid's Disk Wars competition. The Spirit emblem on his back is incorporated into the space of his Identity Disk.
While using the power of the darkness, Riku was capable of transforming into a dark version of himself and use his dark power against his opponents. In this form, his torso, arms, and legs are covered in black and purple material that resembles muscle tissue. On his chest is a symbol of the Heartless, except that the cross is not there, and he has a white cloth around his waist.
Riku gained immense power through the forces of darkness. But ordinary humans are no match for the darkness, and it is gradually consuming his heart.
Riku-Ansem is the form assumed by Riku throughout Kingdom Hearts 358/2 Days and Kingdom Hearts II. He appears identical to Ansem, Seeker of Darkness, wearing a black coat, gloves, and boots. Prior to his battle with Roxas in The World That Never Was, Riku wears a black blindfold to suppress Ansem's influence.
After the Kingdom Hearts Encoder exploded while attempting to turn Kingdom Hearts into data, Riku was purged of Ansem's influence and he transformed back into his normal form and discards the blindfold.