Characters all have a separate Skeletal Mesh asset used to show an Xray visual of their bones or insides when they are zapped by electricity.
It is not strictly required but to add the effect to your character you must bind a mesh to the same bones as your main character mesh and import it into Unreal as a separate Skeletal Mesh asset.
This should be a lightweight mesh, for reference the base cast Zapped Skeleton has around 2600 triangles.
Assign it to the “Skeleton Mesh Component” in the RivalsCharacterRenderer Blueprint.
You can right click – Asset Actions – Export Clairen’s SK_Cla_DefaultZappedSkeleton and use it as a starting point in order to build your own, it uses black and white vertex color instead of textures and should have the MI_Cha_ZappedSkeleton Material instance applied.