How to pipe stdout while keeping it on screen ? (and not to a output file)

I would like to pipe standard output of a program while keeping it on screen.

With a simple example (echo use here is just for illustration purpose) :

$ echo 'ee' | foo
ee <- the output I would like to see

I know tee could copy stdout to file but that’s not what I want.
$ echo 'ee' | tee output.txt | foo

I tried
$ echo 'ee' | tee /dev/stdout | foo but it does not work since tee output to /dev/stdout is piped to foo