From the Super Mario Wiki
The Armored Harriers are ranked 10th in Glitzville's Glitz Pit. They hold a grudge against Mario after Grubba tells them some trash talk that he accredits to Mario (namely the reference to outies running in the family, since their mother apparently had an outie). The Armored Harriers are made of iron that Mario cannot break through. He is only able to defeat these two Iron Clefts with the help of Mini-Yoshi, who is able to take an enemy into his mouth with his tongue and spit them back at the other enemy with his Gulp attack. One of the Iron Harriers wears red shoes, while the other wears green shoes. This combination of red and green appears often in the Mario franchise.
After Mario and his allies beat the Armored Harriers, they are downgraded to the Minor League. They form a grudge against Mario and attempt revenge by ambushing him after he reaches 7th rank.
When Mario has to visit the Minor League locker room while searching for the Dubious Paper, the Armored Harriers threaten to fight outside the ring.
Despite the twins' grudge against Mario, they, along with Jolene, KP Pete, Rawk Hawk, and a few others cheer him on in his battle against the Shadow Queen. It seems though that they let the grudge go, realizing Grubba lied, but still see Mario as a rival.
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