.rcd Files

.rcd files are the files used for the Wii Sports Miis. Each Mii uses a different .rcd file.

Homebrew Method
If you want to make Mii .rcd files with your user-created Mii characters on the Mii Channel, this is a helpful guide for you. First of all, download SaveGame Manager GX for your Homebrew Channel, then put your SD Card/USB Stick into your Wii and make sure you have the mii file you want stored on your Mii Channel. Then open up SaveGame Manager GX, now click on the 'Mii' tab (SD Card/USB Stick icon) and find the name of the mii you want to transfer to the SD Card/USB Stick. Click on 'Extract' then click 'Custom' then choose the folder on your SD Card/USB Stick that you want to transfer the Mii to. After that, you are pretty much done with the Wii stuff.

Now put your SD Card/USB Stick into your computer and locate to where your Mii file(s) are. Once you have found your Mii file, you will see that the file extension is in a .miigx file. All you need to do this change the file extension from ".miigx" to ".rcd".

No-Homebrew Method
Download My Avatar Editor from the Discord server. Then use the My Avatar Editor to make your custom Mii. Once you've made your custom Mii, choose to save it as a .mae binary file. Locate the .mae file you just created in File Explorer, then change the file extension from ".mae" to .'.rcd".