Honestly, I feel these misunderstandings in regards to Kiritsu and Tsubaki are completely justified. He has only seen her with other women before and is more known for her misandry, so his two thoughts when he saw her with another man is that she either is trying to manipulate him like she has done in the past, or that none of her feelings before are genuine and Chihara is either also a tool to her or is being manipulated by her as well. Yet, when she denies it, and looks at him with remorse, it's natural his first intuition would be that he's wrong and she geninuely is close to Chihara, yet feels some envy that she's just using him as a "target." Interesting. (Though, "target is a double entendre" as they're talking about two different things, being blackmail victim vs love interest) Obviously he doesn't have a full picture because she won't tell him how she feels, or she plays it off as if it's a joke or a way to get back at him. She's dug herself into this hole and she can't get herself out because of her past misdeeds, and so she has to atone for them. That's why I don't find it frustrating because it's not a misunderstanding from circumstance, but one that developed because of her own choices and her own actions that are reflecting back on her.
Also, who is Chihara trying to romance? Is it the fujoshi girl, or...?
I'm interested to find out about his character