John seems to be a man discontented with the drudgery of his routine life. He comes home from work with "gloomy and downtrodden cynicism", knowing exactly what to expect when he gets home, dreading it. But when he gets home and she is gone, "it left him dazed." He experiences a "queer feeling of desolation in his heart." This is hard for him, because he is "not accustomed to analysing his emotions". He resolves to be a better man, to treat Katy better. However, as soon as she gets back, he reverts right back to his old ways, and goes out to be with his buds for the evening. So, John seems like a man more comfortable playing the role of the bored and discontented husband; he uses it as an excuse to not analyze his weaknesses or to change. Instead, he blames his misery on other people; hence, he can do what he wants without feeling guilty. When faced with chances to change, he is full of good intentions and no follow-through. He really doesn't want to change.
Katy is harder to characterize because we only get John's perspective of her. She seems feminine, caring, possibly a bit of a nag since she is "wrathful" about his evenings out, and expressed "ire" when he returned.
The two seem to be going through the motions but missing the mark, and don't seem destined for much successs together as a happy couple.
John Perkins is the protagonist of O Henry's story, and I think it is he rather than Katy, who undergoes the entire epistemological process, narrated in the story--from routine monotony to its rupture in the form of an unusual event and then its return at the end with the restoration of the routine. So, we do not get a lot to conclude about Katy. She is a woman who deserves to be loved and we do feel for her immensely but that is about it I think.
What the story establishes through Perkins's self-corrective epiphany once Katy is not there changing into the same routine-bound machine life being restored the moment she returns, is the deadening function of habit, the resistance of the human mind to change the tried and tested tracks of life and explore the unusualities. It also shows us his inflexible nature as a man who is a product of capitalist civilization--a human robot hybrid as it were.