This article is brief.
The Waikiki is the internal codename for the official Nintendo EXI-to-USB adapter used for debugging GameCube and Wii software. The adapter has an end which plugs into the GameCube/Wii's EXI bus via the memory card slot, which then allows for a USB cable to be plugged in which can interface with special PC software to provide debug functionality.
The homebrew community recreated this device under the name "USB Gecko", however the USB Gecko does not have the same internal design or functionality as the original Waikiki, and software meant for the Waikiki is not fully compatible with the USB Gecko.
Waikiki is named after a neighborhood in Honolulu, Hawaii, possibly due to the high presence of Dolphins there.
The Waikiki dongle was used during internal "lab testing", as well as factory initialization, of the Wii and possibly the GameCube. RVL_DIAG contains a program used to test the Waikiki's operations, as well as a serial number registration program which uses the Waikiki and PC software to register a console's serial number.