He basically invented Luigi’s personality on the fly.
Luigi was just green Mario, right until they put Martinet with a live 3D face animation rig at an event and “talk to Luigi!” was one of the options.
He then came up with Luigi as the slightly cowardly, not as confident, but good spirited and kind, companion to Mario. The voice tone, the mannerisms, he just made it all up and Nintendo loved it.
Without that, no Luigi’s Mansion and the green Mario dude would still have no personality.