How to display characters on window without border like shimeji

Basically I’m working on a program that runs unreal in its window. However I want the character in that window to be the only thing shown not the background of the unreal window. I would like this to be similar to what a shimeji program does. For those who don’t know what that is hers a link to a video about what one is - YouTube. Any help with this would greatly be appreciated thank you.