Erlo Ackerman is the boss of Ackerman & Hook Flour and Feed Mill. Though Mr. Hook's name is also in the title, Erlo is older. He's the one who calls the shots. He's not unkind to Tom by any means, but he never coddles him, either, despite having been buds with Tom's grandfather back in the day.
Erlo also gives the impression of being pigheaded—or, to put it more gently, stuck in his ways. For example, when Mr. Hook wants to put a phone in, there's a whole chapter devoted to Mr. Ackerman's baffled reaction. At one point, he goes off on the concept of the phone operator:
"Turning her damn little crank," grumbled Erlo. Then another idea struck him. "She. She. She. Nothing but she. What is it anyway? A kind of suffragism? Ain't there any telephone men?" (15.33)
And then when the Moucheauds suggest going home early because of a snowstorm, Erlo launches into this tirade:
This mill's stayed open until six o'clock in the winter […] ever since I came to work here, and it's going to stay open till then, snow or no snow, as long as I'm running it. You can't decently shut a business like this earlier unless you advertise beforehand in the newspaper. (12.13)
The tirades are comical—he's not actually mean, just crotchety. And Mr. Ackerman does eventually come around both to the idea of the telephone and to letting Tom start home early in the snowstorm. Moreover, most people still respect him despite his blowhard personality because he gets things done. Nevertheless, he also demonstrates the kind of conservative mentality that sometimes butts heads with progressive ideals (like women's suffrage. That's a good thing, folks).