Some fighters smile when they are hurt in the ring, a defence mechanism designed to convey comfort under pressure.
Robert Guerrero is different. He more than likely smiles when he wakes up and does not stop until he goes to sleep, happiness that is disarming in a business not known to encourage bliss.
But Guerrero seems to be unlike most fighters. While there is no denying he is an excellent operator, a six-times world champion at four weights, he walks like a saint among sinners, a God-fearing man with love in his heart and malice in his fists.
On Saturday night at the MGM Grand, Guerrero reaches for another bauble; Floyd Mayweather Jr's WBC welterweight belt, along with the vacant 147lb championship sponsored by The Ring, a battle overloaded with moral certitude, not all of it justified.
Mayweather's incarceration last year for domestic violence, for instance, against his one-time partner in front of his children, and his whining while in prison about the quality of his diet, confirmed the consensus view that he is a "bad hat", a loud disciple of narcissism, and none of his good charitable works around Las Vegas seems to dent that perception.
Guerrero, meanwhile, arrives in the City of Sin from Gilroy, California, a small farming town known for cattle, garlic and mushrooms, a place Steinbeck might have recognised in The Grapes of Wrath, where religion and hard living rub up against each other on a regular basis.
"There's a new sheriff in town," said Guerrero in the foyer of the MGM this week, bustled by a scrum of cameras and fight fans. Most who had gathered to listen recognised him as the hero in this fight, an almost too-good-to-be-true representative of his hometown and of God. However, he has his own burden of imperfection. While in New York last month, Guerrero was apprehended at the airport after confessing he was bearing a gun, a transgression in that state that, uniquely, carries a maximum of four years in prison.
So, 10 days after he fights Mayweather he must fight the law, and there is a chance he could go to prison as the new world welterweight champion, an irony only boxing could accommodate without laughing its head off.
Mayweather has been quick to seize on the anomaly, and has taunted Guerrero, a keen supporter of the TV evangelist and former Republican presidential hopeful Pat Robertson, as "such a hypocrite".
Guerrero says he is not bothered: "We're all hypocrites, man. There ain't no perfect man on this planet. For him to call me a hypocrite, it makes me laugh because it shows me how much he knows about the Bible."
The fact he will earn around $5m (£3.2m), a good $35m less than the blinged-upchampion, and by some way his biggest cheque for fighting, also soothes the deal.
American boxing overflows with praise-the-Lord fighters, to the point of parody. Guerrero, though, has refused to comment on his association with Robertson and membership of the "700 Club", which entitles him to throw money at Robertson and his missionary work.
He is, he says, here to fight, something he has done for 12 years to pleasing and profitable effect, losing only once in 35 paid bouts. That setback arrived in 2005 when, at the suggestion of his promoters, he briefly split with his father, Ruben, to fight the undistinguished Gamaliel Díaz and turned in an anaemic performance. He rejoined Ruben immediately and beat up Díaz with considerable relish in the rematch six months later.
Guerrero's boxing skills were honed by his father in a barn on a potato farm in Gilroy, where his Mexican family have lived for three generations, and where he lives still with his childhood sweetheart, Casey. Every day, he talks on the phone to Casey and their two children.
And here the Guerrero fairytale twists. Mayweather this week decided to goad Guerrero by saying he was milking Casey's long battle with leukaemia to garner sympathy and support in the lead-up to the fight. "I think trying to gain fans by having a sympathy story every week … I don't think that's a good thing," he said, adding, "but I'm glad that his wife was able to beat the leukaemia. I don't feel nobody should go through a situation like that, but we all go through certain things. Our mothers, our fathers, our loved ones go through certain things. I just feel like … just to gain fans you are using your wife's story, you are using a sympathy story."
The feelgood story now had an edge it lacked. It wasn't just the new sheriff arriving to gun down the bad guy. It was personal beyond hype. Mayweather may come to regret those remarks, however honestly expressed.
The backlash came on Wednesday, when Ruben launched an oath-strewn rant at Mayweather during the final press conference, screaming that he was "a no-good wife-beater", demanding to know why anyone should respect someone who "beats up a woman in front of his own children". Mayweather remained silent throughout.
For all his smiling facade Guerrero is not unfamiliar with the game's dark arts, and, in the heat of the contest, he will no doubt remind Mayweather of his expertise.
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