WARNING: The following contains spoilers for Aggretsuko Season 3, now streaming on Netflix.
Over Aggretsuko's three seasons, Retsuko has had a few different love interests. She dated Reasuke in Season 1 and Tadano in Season 2, chosing to break up with both of them. Then there's Haida, her co-worker who's been pining after her for years. While Haida told her how he felt at the end of Season 1, she turned him down because she wasn't ready to be in another relationship. That may have looked like the end but even in Season 3, Haida can't seem to get over Retsuko.
Although the season finale left their relationship ambiguous, it's about time these two got together. Retsuko hasn't given him a fair chance in the past, but if she did, she'd probably find that they're more compatible than she previously thought. Now that everything's out in the open, maybe they'll finally start dating.
One of the main problems with Retsuko's past boyfriends was that they didn't know "the real Retsuko," as represented by her death metal persona. It's not a side of herself that she likes to share with other people, so it's a big deal when she chooses to reveal it. Her relationship with Resasuke was pretty one-sided from the beginning; when she opened up to him, it became clear that he didn't understand her at all. In Tadano's case, she used music to tell him that she wanted to get married and start a family. Sadly, he didn't want the same thing, which was a deal breaker. Both boyfriends just weren't right for Retsuko.
Conversely, Haida himself has admitted that he doesn't know Retsuko as well as he'd like to think. Instead, he fell in love with his idea of her. Despite his self-awareness, Season 3 called this into question when Haida tried to explain to Fenneko what he liked about Retsuko. He recounted a story about how he found out that she was stealing water from the cooler at work to take home. This showed him that Retsuko wasn't just a perfect little worker bee doing whatever she was told, and this was the first of many things that made him like her. They may not be close, but Haida has been observing her for a long time and he knows her better than he thinks.
Tadano may have rigged Gori's app to say that Haida and Retsuko were 100% compatible, but out of her potential love interests, Haida does have the most in common with her. They've worked together for five years, and while they've never talked about kids, he was quick to respond when she brought up hypothetically marrying him. Most notably, he's also into death metal. Every season has ended with Retsuko taking someone to karaoke to reveal her true self through death metal music. In Season 3, Haida flipped the script and took her to karaoke so he could belt out his unfiltered feelings. If he wanted her to be honest and open with him, he had to be just as open and even brutally honest.
While many have taken issue with the way Haida handled Retsuko's trauma from being stalked and attacked, the show had limited time to dedicate to her recovery. It's unclear how much time Retsuko actually spent at her parents' house after the attack, but it had been long enough for Washimi to worry that the company would fire her if she didn't return soon. She likely had more time than was shown. Moreover, when Haida took her out of the house, he brought her to the one place where she'd always felt safe being herself: the karaoke bar. He couldn't have expressed himself anywhere else and neither could she. Finally, as the show has told viewers time and time again, Retsuko bottles things up and pretty much never asks for help. It may have been an aggressive move, but Haida knew that if he wanted to really be there for her, he had to step up in a big way.
In the end, Retsuko did open up to Haida. He was taken aback at first, but after listening to her vent about him and about life beating her down, he took her hand and told her, "Let's go punch them back!" Unlike her past boyfriends who have witnessed Retsuko's rage, Haida didn't try to argue or just passively observe. In contrast, he actively joined in and supported her. He understood her and was able to meet her at her level in a way no one else had before. Retsuko may not have seen Haida as a viable romantic partner before, but her view of him is changing for the better. If Aggretsuko does return for a fourth season, it should definitely continue to explore Retsuko and Haida's relationship.