Unpopular opinion: The Boruto writers could have created an INCREDIBLY CUTE couple story with Buntan and Shizuma, but they were too lazy.
Buntan was loyal to Shizuma until the end, and even when they lost to Boruto and his team, she remained by his side as a good friend/girlfriend. She only abandoned him because of Shizuma's betrayal in trying to escape from prison (even I would have given up!), and that's when everything went wrong!
They could be villains, whatever, but the problem was never the couple; they just wanted to make Shizuma even more of a villain than he already is, without even creating a decent script explaining why he's a villain. Making a villain a son of a bitch all the time doesn't make him a developed character, it makes him a hateful character.
Okay, I would understand if Shizuma had difficulty showing feelings, since the Hoshigaki Clan abhors feelings and anything that makes someone seem weak, but Shizuma didn't seem (in my opinion) to have difficulty showing feelings, much less be a toxic boyfriend. I KNOW that everything he did, acted, and said to Buntan was fake, but if you ignore that detail, he was a caring person. He listened to Buntan, stayed by her side, was sweet, had all the characteristics of a good boyfriend, but the writers RUINED EVERYTHING!!!
I suspect the entire Naruto/Boruto franchise has difficulty creating truly good couples, I don't know why.
These two give off a shark-and-ray vibe. The shark symbolizes strength, protection, family, and ancestry, while the ray symbolizes spiritual protection, wisdom, adaptability, and emotional healing.
Buntan resembles the ray in her ease of adaptation; when she and Shizuma end their relationship, she easily adapts to life, leaving him behind and becoming a better person. Shizuma, representing the shark (obviously), has a connection to ancestry due to his plan to bring back the ancient times of the Bloody Mist Village and his physical and mental strength to withstand so much pressure.
(In my head, it makes sense.)
Spiritually, the ray-shark relationship is often seen as a balance between brute force and intuition (shark) and wisdom and fluidity (ray). Shizuma blindly follows his intuition and relies heavily on brute force, while Buntan is more intelligent and adaptable to the environment. Even after the fight with Sarada, both demonstrated mutual respect and good teamwork after the battle.
I like their aesthetic: boyfriend with long hair and girlfriend with short hair.
Instead of Buntan, it's Shizuma who spends more money on hair care, especially on her curly hair. Buntan would be too lazy to take care of her hair, so whenever she can, she cuts her blonde hair.
They give the impression of being a couple who listen to TV series theme songs, are fans of forgotten movies (like "The Thing" or bad comedies that only they find funny), like to talk badly about rich and privileged children, and are a passionate couple.
If Kisame or Itachi were part of his life, he would constantly ask them for love advice, telling him how he should act or asking if he had been a bad boyfriend for acting in a certain way. I think Itachi would be the one who would help him the most, giving advice on gifts, what to do, etc. Kisame, on the other hand, would only offer moral support, I think.
Kisame highly values loyalty, truth, and trust in a relationship, and I highly doubt Shizuma would think differently in that aspect. Shizuma would also be very loyal, just like Kisame.
(It's enough that boys treat pretty girls badly, it's enough that Sasuke does it!!)