"She comes off as -- everybody know this -- very B-I-T-C-H-Y," Jamie admitted to Derek.
"She came off as a brat," Doug interjected.
"She does kind of come off as a brat," Jamie agreed. "Do you think that could be due to editing or do you think she really is -- I don't want to say cold and harsh, but it seemed like she comes off that way a bit. Do you think she genuinely is like that in real life? Was that your experience?"
"Or was it just towards you?" Doug added.
Derek responded, "I believe that it is not just towards me, although I do believe it was a little bit more concentrated on me. Granted I was the focal point for a lot of it. I do believe Katie is a bit rough around the edges."
Derek, however, avoided using the offensive word b-tch to describe his former wife.
"I do not think that she is a, um, the more meaner word for 'brat,'" Derek said.
"With a lot of things, I believe it was the culmination of a lot of pressure. And a lot of things that she lashed out about were things we had talked about already that had bugged her. Some of her biggest lashouts weren't just out of the blue, something led up to it."
Derek was then asked on Hot Marriage Cool Parents how he felt about Katie getting jealous of a date he had planned for a woman in college, which included the spontaneous event of sneaking into a circus.
"It was for a fraternity formal in college, I believe my sophomore year. But it was a little odd how she reacted," said Derek, who once wrote a love poem for a girl that also pissed Katie off.
"I was really expecting her, I mean, to see the poem and finding out about the cooler and sneaking into the circus that it would've made her see better of me. [I thought she'd] see me in a better light."
Derek then went on to reveal what he considered to be Katie's "biggest issue" in their marriage.
"The biggest issue Katie had in our marriage was the fact that from just a couple of days in [to the experiment], she built a label for who I was, and any time I did something outside of that label, it threw her off and made her feel like she didn't know me at all," Derek explained.
"And that at times was very frustrating, and I was hoping that her seeing this side of me would allow her to peel back that label or maybe edit it, but it just made her lose all trust in, more or less, my identity or what she thought I was."
Derek continued, "I didn't really even know she was that hung up on it either. Even the poem, almost a week later when we were going to the retreat, I had no idea that was still on her mind, and I only found that out through the show."
Derek called the situation "weird" since it was just an old poem Katie had discovered while going through his childhood belongings through boxes in his garage.
"I feel like I almost wish I didn't show her all of my childhood stuff because of the way she reacted. I don't really know what else to say that," Derek noted.
Doug agreed it was "strange" because Katie took jealousy to a different level.
Although Derek and Katie opted to stay married on "Decision Day" at the end of Married at First Sight's eight-week experiment, the couple revealed they had split during the reunion special, which aired in late April on Lifetime.
Katie complained Derek was "fake" and her marriage lacked physical intimacy because Derek allegedly never initiated in the bedroom, but Derek vented about how Katie had allegedly cheated on him twice with her ex-boyfriend -- once after their honeymoon, and then again about two weeks after "Decision Day."
Katie admitted to having a physical "slip-up" with her ex after "Decision Day," but she completely denied ever having an affair once she and Derek returned to Washington, D.C. from their honeymoon in Panama.