Ever since Azula was imprisoned, she's spent most of her time reflecting. Mainly reflecting on the fourteen years of her life and how she got to be where she is now. It's not like there was much else to do in her cell. The main thing she found herself reflecting on was her position in the war. There was no denying her involvement but it was over now, so all she could do was think.
What was she even trying to do? After her time spent behind bars, she began to realize that she didn't even care about the Fire Nation much anyway. She only cared about people, very certain people. Her parents, Zuko, Ty Lee, Mai, Iroh, all of them. She let all of them into her heart, fought tooth and nail to get to the end goal, and was now the farthest away from anything. She fought for power but now had to beg for control.
She wished she could start over. Do it again from the beginning. She would go with Zuko, she would join the Avatar, she would've loved her friends more than feared her father. She would've lived in love and felt fear. But it wasn't meant to be. Now she was stuck here, probably for the rest of her life. No one but the dank walls and occasional visits from her brother, who was now the Fire Lord.
At least, it was usually her brother, but recently someone new would also visit her cell. The water tribe girl, Katara.
Azula still remembered the first time Katara came to visit her and it was strange. The weight of the world seemed to increase tenfold when she saw her face, panic set in as the events of the Agni Kai played over in her head again. The way she was so easily overpowered, completely frozen in a block of ice in under a second, then found herself chained the second she came to senses. That was terror, pure terror, and Azula was now at a point where she would admit that. She was scared of Katara. Katara made her feel like prey trapped in a cage, as the predator would watch. She was able to overpower back then and who's to say that one day she wouldn't want to finish the job?
It's for this reason, Azula barely remembers their first interactions. Katara would stop by about once a week, maybe to check up on her or get properly aquatinted. Again, Azula barely remembers their first interactions. But as time passed more and more, Azula started to allow herself to open up to her. If she was going to regain some control over her life, she had to give even Katara a chance. She was already living in the worst case scenario.
After allowing herself to open up to Katara, she slowly began to feel a bond to her. Katara was actually quite an interesting person. She had plenty of entertaining stories to share with her about her time travelling across the world and about Toph, the blind earth-bender. Every time Azula heard Katara's stories, she started longing for that type of life. Azula could've had Katara's life, she could've been able to enjoy these peace times with Ty Lee and Mai right now if only she had gone with her heart.
Azula wanted to wallow in this thought but that was when Katara said something to her that stuck to her. "Azula, you're allowed to be angry." Azula didn't want to answer at first. Her conversation skills were already bad before she was imprisoned but now they've only gotten significantly worse. Best response she could muster in this situation was just looking at her in the eyes and trying to nod appropriately to what she was saying. "This isn't fair. Everything that happened to you wasn't fair and you're allowed to be angry but just make sure you're not angry with yourself. You're changing and that's good."
A wave of emotions washed over Azula. A wave of anger, a wave of sadness, a wave of joy, all of them crashing over her at the same time. What should her response to that be? She would be stuck in a damp prison for a war that was started before she was born. A war that she lost. A war that she experienced loss. But also, a peace time that let her talk to Zuko again. A peace time that let her potentially start a new friendship with the one who defeated her. Everything was confusing, everything was changing, everything was the same. Just a few sentences and Azula's head was spinning.
"I'm sorry, Azula, I didn't mean to upset you. I'll-"
"No," Azula's voice was slightly hoarse due to amount of talking, or lack of, she was used to now, "I appreciate it." What she said next surprised even her but it shows that she was changing. "Thank you, Katara."
With that Katara gave her a smile.
As time pasted, along with introspection, Azula has also started dabbling with poetry. She wanted peace and her uncle seemed to have achieved it, copying his perspective on life seemed like a good way to go. She was living in darkness but it didn't matter how long or prominent the darkness might be, even a flicker of light would be enough for her. It would help her out of her own darkness. At least that was what she told herself. It seemed too on the nose but this was just for her.
As she thought more about any other potential poetry she could tell herself, her cell opened and it was Katara, as she expected. If there would be any light in Azula's life, she wanted it to be Katara. Ironically Katara was what fire should be, tender and warm, not destructive. She did still embody the water tribe though, she was gentle but also she was light. Light in a way that fire wasn't supposed to be. She wasn't the sun, she was the moon. She rose when times are darkest and reflects the light of those around her.
Maybe that was Azula was trying to get at earlier. Light always exists and right now, in the depths of Azula's darkness, Katara was the light. Katara was her moon.