Especially in the Paper Mario games. Peach is obvious, but at this point we have pretty much every female partner. He's relatively short, somewhat overweight, has a massive nose and head, and...it's the mustache, isn't it...
Well, the mustache is one thing, but it's also because he's a hero and he's willing to help them.
Protecting Goombella from Crump and the X-Nauts.
Finding Flurry's necklace.
Finding the Superbombomb for Vivian and cheering her up when she discovered the bomb was broken even though he had way bigger problems at the time, (trying to get his identity back from Doopliss), he puts other people ahead of himself.
I think that all the Paper Mario's and M&L:SSS all took place before Luigi's Mansion since Luigi lived with Mario in them and didn't move out until he found out he won a competition that he never entered.