Walter Parks Thatcher is basically the ultimate businessman in this movie. He's everything that Charles Kane has grown to hate, the kind of dude who will always protect the interests of the wealthy class by stomping on the poorer classes.
Thatcher actually acts as Kane's legal guardian for most of his young life, but at the end of the day, the dude's an accountant and not a father. We never get a look at the years Thatcher spends as Kane's guardian, but we get some insight in Thatcher's journal when he says,
"THATCHER: He was, I repeat, a common adventurer, spoiled, unscrupulous, irresponsible.
Then again, this might also just be hindsight speaking, since Kane grows up to stand for what Thatcher hates most: the rights of the workingman.
Thatcher must not have been a very attentive guardian over the years, because he seems totally shocked when Kane first takes control of his family fortune and tells Thatcher,
KANE: I'm not interested in gold mines, oil wells, shipping, or real estate.
Thatcher coughs and yells, "Not interested?!" because he can't seem to believe Kane would care about anything other than his money and property.
Once Kane becomes a young man, Thatcher tries his best to convince Kane to act more like a normal rich person. At one point, he even asks,
THATCHER: Tell me honestly. Don't you think it's rather unwise to continue this philanthropic enterprise, this Inquirer that is costing you $1 million a year?
What he doesn't seem to understand is that Kane believes in something much more valuable than money: his own ego. And that means he's willing to bankrupt himself just to make life hard for America's richest people. No matter what Thatcher says, he will always be the enemy in Kane's eyes—the type of "money-mad pirate" whose only concern in life is to become wealthier.
By the time he's an old man, Thatcher is willing to criticize Kane publicly by saying,
THATCHER: Mr. Charles Foster Kane, in every essence of his social beliefs, and by the dangerous manner he has persistently attacked American traditions of private property, initiative and opportunity for advancement, is in fact nothing more or less than a communist.
Calling a dude a communist back in those days was no small accusation, especially from a dude who has spent over a decade as Kane's legal guardian. But Thatcher's loyalties are with his money, and that's why he takes on Kane whenever he can.