Here is a design suggestion.
First, one of the 6 icons is selected. These are Set default athlete, new athlete, delete, copy, backup, save.
After selecting an icon, the square appears in the athletes.
The green ticks indicate which athletes have been selected and perform ends the action.
For backup and save select all can be used and for the others not.
Nice to have but not important. If the default athlete is always at the top left.
I don't know if this is easy to implement.