The internal tolerance gauge came early for Otto Wallin, and sometimes accompanied with teary eyed pain. But that’s what older brothers are supposed to do, aren’t they? Pick on the smaller, younger one when they have the chance, before the younger sibling grows a little too big and gets a little too strong to realize that baby brother isn’t going to be running to mom crying, but rather running through you.

Somehow, someway, Petter and Marten were going to find new, and imaginative ways to twist their younger brother, Otto, into a pretzel growing up as kids. Petter holds an eight-year edge over the baby of the family, while Marten is four years older.

The trio play fought a lot—with the younger Otto getting his ass torn up.

But time has a way of serving as a great equalizer. It eventually balances things out, and in many cases, makes the smallest and youngest into the fiercest of the household.

Little bloody Otto, through phases, became the terror.

Then, it became time to unleash Otto on everyone else.

That’s what is happening now as Otto Wallin (20-0, 13 knockouts) builds himself into a heavyweight contender. Otto, 28, a 6-foot-5½ southpaw from Sweden has been living in New York City the last two years training under former WBA super featherweight and lightweight world champion Joey Gamache.

Otto will be making his U.S. and national TV debut against Nick Kisner (21-4-1, 6 KOs) in a 10-round undercard bout Saturday night (9:10 p.m. ET/6:10 p.m. PT) on “Showtime Boxing: Special Edition” tripleheader from the Adrian Phillips Ballroom in Boardwalk Hall in Atlantic City, featuring the undisputed women’s middleweight title fight between Claressa Shields and Christina Hammer.

Petter and Marten will be watching from Sweden, while his father, Carl, will be ringside. It was Carl and Petter, 36, who got Otto into boxing when he was 15. Otto played ice hockey and soccer growing up in Sundsvall, Sweden, but it was dad’s amateur past and Petter’s combat sports pedigree that set a high bar for Otto. Petter was a silver medalist at the 2010 Muay Thai boxing world championships in Thailand.

“I fell in love with boxing right away,” Otto said. “My father had around 20, 25 amateur fights and he still trains people today. Petter also had around 20, 25 amateur fights. I’m the youngest of three boys. Petter was an inspiration for me growing up. I always wanted to be as good as him. I learned how to keep it together when they were hitting me.

“It wasn’t like they were abusing me or anything, but I was always the younger and smaller than them. Now, I’m the tallest. I’ve done some sparring with Petter, but I wouldn’t risk it too much with Marten. Petter is still competitive. When I got older, I didn’t get picked on that much. But it didn’t stop until the age of 20.

“It’s still not easy. Marten does jiu-jitsu, so I have to hit him first, and Petter has all of these Muay Thai moves. If they get a hold of me, it’s over. It’s pretty tough competition in the house and that’s helped me a lot. It’s played a big part and my father being a trainer has helped a lot.”

This is the first time Otto will be fighting in a year. He’s had fights cancelled and he’s looking forward to get back into the ring three times this year.

On paper, Otto should have an easy time against the 5-foot-11 Kisner, who’s been stopped once.

“I know he’s about 6-feet, smaller than me and he tries to be a cutie, and he’ll try to be tricky,” Otto said. “Joe (Gamache) has watched him a lot, and I feel we have a good game plan. It’s important for me to put in a good performance to show Showtime they made a good choice in putting me on.”