Thinking about potential mario canon where mario is just like. Peach's bodyguard. It pays well and he gets staff meals (cake)
No listen it makes sense. They have a friendly professional relationship, first name basis, been working together for years, but they're still somewhat cordial since she IS a princess. After the first time he defeated bowser he was brought on permanently. This is why he comes with when Peach goes on vacation in sunshine, he's her security staff
Luigi doesn't clock in as often anymore ever since his ghost hunting/real estate business picked up which is why he's absent from most 3D games but he is still on staff
luigi is helps keep the mushroom kingdom in good relations with the much larger and more powerful sarasaland empire.