They only ever claimed to be district partners, but it is the unclaimed thoughts and feelings that matter the most.
Cato would have volunteered anyway, but when he saw Clove walk onto the stage as a tribute, he was overwhelmed by the desire to protect her. She reminded him of his sister, not his older sister who died in the Hunger Games, but his younger sister who died in a training accident.
Clove saw in Cato the one thing she had always wanted but never had: a brother. She always wanted a brother that would push her to her limits. A brother to compete with, one that she would always strive to beat. She never wanted a sister, because boys were naturally stronger, which is why she wanted a brother: to challenge her.
They started their alliance on the train. Cato could not help but recognize the similarities between Clove and his dead younger sister; they both had a competitive attitude and the pride to go along with it. Clove's eyes, if he was not mistaken, were the same shade as his sister's; and there was no denying that the scar along Clove's jawline, that would most certainly disappear after she was "remade", was strikingly similar to the one his dead sister wore.
Clove couldn't help but notice how identical Cato was to the brother of her dreams. He had the same haughty arrogance that only made her want to beat him more. He was much taller than her, too, which was how had she wanted it; and judging by his muscles, he had exceptional strength, which only made the feat of beating him greater, and that much more rewarding.
Cato was impressed by Clove's skill with the throwing knives, and sure enough, that was his younger sister's weapon of choice, too. The skill she posessed was equivalent to his sister's, just another remarkable similarity between them.
To say Clove noticed Cato's swordmanship would be an understatement. She was enthralled by it. His swordmanship was just as good if not better than she had imagined. The brother she had always dreamed about wielded a sword, and Cato, she thought, was that brother, even if not by blood.
They grew closer, though not intentionally, through their heated arguments, playful banter, and mutual hatred towards the girl who stole their glory during the tribute parade, the girl they called Smoke Girl, because now that her fire was out, she was nothing but the smokey remains of what she could have been (or at least they thought).
Cato's training score was higher than Clove's, only by one, but it was still enough to make her furious, envious, and, if it was possible, more competitive. He made sure to point out his higher score, because he knew it would infuriate his sister, just like it did. What infuriated both of them, though, was the eleven score of Smoke Girl, who now became Fire Girl, because she wasn't a disappointment like they had thought.
During the Games, Clove tried her best to one-up Cato. She almost killed Fire Girl on the first day. She was planning on killing her as another way to beat Cato, but Fire Girl had got away. So, all she could do now was kill more tributes than Cato during the battle at the Cornucopia.
Cato did what he could to protect Clove during the Cornucopia battle. That's why he had killed one of their alliance members, because he thought that the guy from District Four was going to kill Clove, so he threw a spear into Four guy's chest. No one saw this kill, they were all too busy with their own, so he used another tribute as his scapegoat.
Cato couldn't protect Clove from everything, though, so when the tracker jackers came, all he could do was yell, "To the Lake!"
The venom from the tracker jackers caused Cato and Clove to see delusions. Clove hallucinated about losing the Games, Cato dying, her parents dying, and herself dying by Cato's hand. Cato relived his sister's deaths, and saw Clove and him dying. The unknown time they spent seeing these things were one of the worst moments of their lives.
They fell right into Fire Girl's trap, and when their supplies were blown, Cato lost it. Why did it have to be so hard to protect Clove? Clove had to calm him down. That's what a sister would do, isn't it?
The rule change that allowed for Cato and Clove to win together was a bit of a disappointment to Clove, but she was more glad about the rule change than she was disappointed by it. She wouldn't be able to beat Cato in the Hunger Games now, but she finally had a brother and she didn't want to lose him.
The rule change affected Cato greatly, he was already planning on letting Clove win, but now all he had to do was protect her, which he was doing all along. He didn't have to die. He could keep her.
Things went horribly wrong at the feast. Clove was supposed to kill Fire Girl, and Cato was supposed to stay close by to ensure nothing went wrong. Cato got distracted by a tribute he only saw once before. If he could kill her now it would save time later, and Clove and him were so close to winning.
Clove was going to savor the death of Fire Girl. The game Fire Girl was playing with them would end now with her own game. How much pain could Fire Girl stand before she passed out or died? Things went horribly wrong, though. The huge tribute from District Eleven picked Clove up off of Fire Girl. He had a rock, and Clove, fearful of death, called out to Cato. The rock smashed down on her head, but not until she heard Cato return her call.
Cato heard Clove's call and ran back as fast as he could. How could he have lost sight of protecting Clove, the one thing that mattered? How could he have been so stupid? When Cato saw Clove on the ground dying, his heart broke. He couldn't lose her, not if they both could win. He had already lost her once, he didn't want to lose her again. He begged her to fight through it, to stay alive. If she could hold on, then he could kill the other tributes and they would win. They could go home together.
The words that came out of Clove's mouth were jumbled and incoherent. What she was trying to tell Cato was that he was the brother she had always wanted, but never knew she had.
Cato held Clove in his arms while her cannon fired. It hurt just as much as the first time he lost her, and then some, because this time it was his fault she died. He vowed to avenge her, and he did. He tracked down her killer, or Dead Man as he liked to think of him, and used her knives as an instrument for Dead Man's death.
Cato tried to win the Games for Clove, but the muttations were too over-powering. He couldn't hold them off forever. When he fell to the muttations, they bit at him and ate parts of him, but not enough. Cato was still alive when they dispersed. He had to crawl to the opening of the horn if there was any chance for him to win. When he saw Fire Girl, he knew he wouldn't be able to kill her, so he hoped she would kill him. That he wouldn't have to feel the pain anymore. Fire Girl sent an arrow into his skull. He could be with his sister once again.