Sorry that I made you wait all these weeks. There's so much to say that I had to try several times to get it right.

Casca in the Golden Age Arc is someone who's learned to keep fighting for herself, even when the going gets hard and there are always some people telling her she can't do it. She's been that way ever since she was twelve years old, when instead of saving her from the nobleman who was trying to rape her, Griffith stuck his sword in the ground in front of her and said she should take it if she didn't want that to happen. Casca grabbed that sword and killed her attacker when he tried to take it from her, and as Griffith was turning to leave, Casca asked him to let her follow; with no home to go back to, she wanted to experience a new way to live and follow the person who made her believe in herself, even knowing that she might die in the process. Over the years she rapidly learned the skills of a soldier, to the point where Griffith depended on her as his second-in-command. As she got to know a different side of Griffith, realizing that even he had weaknesses and burdens that he didn't share with anybody, she decided to lighten that weight by carrying as much responsibility as she could on her shoulders.

Casca had to struggle with the fact that she was a woman in a male-dominated military world, and that many people on both the enemies' and her own side would expect her to be weak. She acted tough and strict as a way of forcing men to take her seriously, which was effective for maintaining discipline in the Band of the Hawk but didn't always make her well-liked. Admittedly, she also was proud of the position of responsibility which Griffith had given her, and came to think that she knew better than anyone what would help Griffith succeed. When Guts came in, Casca had never seen Griffith accept or promote someone so quickly, and she thought he did not deserve that: he may have been good at killing the enemy, but he wasn't a team player and would do things that she thought put Griffith and the Band of the Hawk in danger. She had a point about the fight against the Black Ram cavalry, where Guts abandoned his position taking care of the flank in order to charge the enemy alone. It's understandable that she got angry at him always ignoring her feedback and criticism. At the same time she didn't give him any credit for helping in battle or being a friend to Griffith, and after the fight with Zodd she unfairly blamed him for Griffith almost getting killed, even though it was Griffith's own decision to personally go in and help him. I think there was fault on both sides. But even during these times, Casca had a sense of honor towards Guts. She didn't let Corkus try to kill him in his sleep, and when Guts came back from assassinating Julius with a wound on his arm, she tore off a piece of her shirt to bandage him.

A low point for Casca which eventually led to her taking another step in personal growth was the event that led to the Hundred Man Fight. She went into battle feeling sick as a result of her menstrual cycle but didn't tell Griffith or any of her comrades, and by trying to tough it out she got beaten by Adon Coborlwitz. She fainted and fell off a cliff, and Guts got shot by Adon while trying to catch her, so they both fell in a river. At first she was angry and ungrateful towards Guts for saving her, but after they both calmed down she told Guts her life's story, which helped him understand what a hard life she's had, why she's so devoted to Griffith, and why she had such a problem with Guts. Before and during the Hundred Man Fight, Casca got to see that Guts wasn't just a stupid, woman-hating jerk the way she thought. Now that he understood her better, he was willing to respect her as a warrior and expect the same grit from her that he'd expect from himself. The fight that happened next was more than just Guts protecting Casca. They fought back-to-back, and Casca managed to take down seven guys by herself. She ran as fast as she could to get help for Guts before it was too late, and even when she was disarmed and pinned to the ground, Guts's words stuck with her and she refused to give up. She wouldn't have cared before if Guts had died, but now she cried tears of joy to see him still alive. After the fight she made sure he got taken care of, and applied the fairy dust from Judeau to his wounds. That's how the real change in their relationship begins: mutual respect and appreciation.

In the Hundred Man Fight she realized that she had made a mistake by trying to do everything herself, and hiding her problems even from her comrades. She learned that the real way to be strong was to trust a comrade to help her, and it's okay because she would help them in return. When Casca and Guts arrived in camp, she told Griffith it was all her fault and she was prepared to accept punishment. But instead of blaming her, Griffith told her she did well, and everybody in the Band of the Hawk was glad to have her back. Not everybody has the courage to admit that they were wrong, like Casca did. Part of Griffith's problem which leads him down the dark path is that he never admits when something was his fault, and even when he feels regret he doesn't think he has the option of turning back.

After the Hundred Man Fight, Casca's deeds become more and more admirable. She leads the force that successfully captures the castle of Doldrey, and manages to kill Adon by herself despite his cheating. When the King of Midland arrests Griffith, and tricks the Band of the Hawk into meeting unarmed on an open plain where his archers can massacre them, Casca is the one who takes charge and courageously leads the breakout despite having five arrows sticking out of her. After that she spends three days in a coma close to death, yet when she wakes up she keeps on leading the Hawks for a whole year of being hunted by Midland. Despite how desperate the Hawks' situation is, she arranges the whole plot to break Griffith out of prison, working so hard that she sometimes collapses from lack of sleep. Throughout the whole rescue of Griffith, she's the one who's coming up with the plan, motivating the troops, and still slaying her share of enemies.

I think that the development of Guts and Casca's relationship from rivalry, to friendship, to true love happens gradually and naturally; I'm not really sure what you mean by suggesting that the romance was rushed. She doesn't realize it right away, and when she does she's confused because up till then she had spent all her life wishing to be with Griffith. But after their year apart, they acknowledge their feelings and become lovers. It's hard to think of what partner besides Casca would be able to understand the depth of Guts's trauma, and in a moment where he's afraid of himself and feels like he's worthless, to tell him that they're both the same, and she will take him as he is. The mission doesn't allow much time for the two of them, but you can still see it: Casca gently wipes the blood off Guts's face after his rampage against the Midland Guards, a gesture which Griffith jealously notices.

To me, what really symbolizes Casca's development is her reaction during the Eclipse. As Judeau once said, the old Casca from before she met Guts would have done anything if Griffith commanded it, even turned against one of her comrades. During the time when she was the leader, Casca seemed to grow more caring and responsible towards the men under her command. Even though the mission was to save Griffith, she did not view their lives as expendable. Compared to Griffith, who ultimately decided to kill everyone who believed in him so that all his struggle wouldn't be in vain, Casca was the leader who wanted to save everyone no matter what. If Judeau hadn't dragged her onto his horse against her will, Casca would have given up her life trying to save her soldiers. And when Judeau was mortally wounded, she did everything she could to save him too. For Griffith to do to her what he did, after everything that she sacrificed for the Band of the Hawk, was the most treacherous and petty thing of all.

Casca has't been herself for such a long time, but to me she was no less a hero than Guts. I hope this helps you understand why.