The formation of their friendship was all her parents' fault, of that she was certain. Her bond with her mother, and his with her father, and her parents' bond since long before she was born or they married, had been the key element needed to start their friendship.
As she developed from babyhood to toddler, she and Tenzin found themselves placed together more often than not, mostly due to their closeness in age. She had little to do with his brother, who was three years older than he and four years older than she. The most she saw of him was his constant pranks and teasing, and the most she heard of him was his vibrant laughter or the adults' exasperated fondness.
She was two when she first saw the looks aimed at the six-year-old Bumi. Tenzin had just bent air for the first time at age three and eight-year-old Kya had daily water bending lessons with her mother. Bumi, however, did not have lessons, and had not shown off any bending at all. It was unusual for him not to show off, or to use his bending to help him pull off his pranks. As the elated hugging died down with the revelation of Tenzin's airbending, Lin saw the looks Aunt Katara and Uncle Aang shot Bumi, and she couldn't help but be curious. They looked the same way that Daddy looked when she called him Daddy, as though they were happy and sad at the same time. She couldn't understand why they would look at Bumi that way, and wondered if Bumi was sick.
Walking home that night, between her Mama and Daddy, she decided to ask.
"Mama, is Bumi sick?" Daddy stumbled, and Mama frowned.
"Of course not. Katara wouldn't have let him play today if he was. Why?" Lin frowned; there went that theory.
"Aunt Katara and Uncle Aang were giving him funny looks, like the kind Daddy gives me when I call him Daddy. And Tenzin was showing off his airbending all day and Bumi hasn't showed off his bending at all and Bumi's more likely to show off than Tenzin is, especially when other people are showing off." She explained, hoping her mother would have an answer.
And that was how she learned that some people could bend, and some people could not. It was also when she found out that, like Bumi, Daddy couldn't bend either. It wasn't until a week later that they sat her down to talk to her about how Sokka wasn't really her father. She had responded with all the childhood naivety she possessed when she told them that of course he was her Daddy because he helped raise her and he played with her and bought her things, just like other Daddies did for their children. When they reminded her that Uncle Aang did those things with her too, she shook her head firmly and told them that it was different. Uncle Aang was Kya, and Bumi, and Tenzin's Daddy. She had continued on until her mother, frustrated, had thrown her hands up and stalked off. Daddy had given her a hug before following.
When she was told that her Daddy was going to have another kid, his 'first real kid' her mother had put in snidely, she had been heartbroken and had run away. Her mother's friend from the police station, who she would later learn was her mother's Assistant Chief of Police, found her hours later, long after it had gotten dark, shaking from the cold and fear, with tear tracks on her face. When he asked her why she had run away, the whole story had come spilling out, about how her Daddy was going to have his own kid and how he wouldn't want her calling him Daddy anymore and how he wouldn't have time for her anymore. Harold, as he said his name was, listened quietly as he carried her home. When they were a block away, and her breathing had calmed, he paused.
"You know," he said quietly. "I have two babies of my own. Now I don't know what your Daddy's like, but me I love both my babies. When my little boy came along, I didn't love him any more or less than I did my girl, and I didn't love her any less now that I had someone else to love. My heart grew bigger so I could love them both just as much as the other, with as much love as I had for one. You understand?"
"So…" she began hesitantly. "You don't think he'll love me any less just because he has another kid? Even if I'm not really his kid to start with?" She asked hopefully.
"I can almost guarantee it." Harold said, nodding. "And I bet you, if you ask him, he'll say the same thing."
"I probably shoulda asked him before I ran away." She admitted sheepishly. "Mama's probably really mad at me."
"Nah, she was just scared something had happened to you." She stared at him, wide-eyed.
"Mama's not scared of anything." She denied vehemently.
"Mama's scared of losing you." Her mother's voice came from in front of them. She spun around to see her Mama and Da-Sokka standing not far in front of them.
"Evening Ma'am," Harold began cheerfully, "I think I have something here that belongs to you."
"Watch it Harold." Mama warned, but there was a relieved happiness there that Lin had never heard before. Harold set her down and Lin instantly darted into her mother's open arms.
"Do that again, and you'll be put in a metal box until you're thirty." Her mother warned. Lin smiled.
"Yes Mama." She agreed. Her mother scooped her up.
"Go back to your family Harold, and thank you."
"Anytime, Chief. Anytime." The man said sincerely. "Bye little miss."
"Bye!" Lin called back as her mother walked back to their house.
She had been treated to a long talk about how she wasn't to run away ever again and how much they had worried. Sokka had tried to talk to her about it, but even with Officer Harold's word running through her mind, her mother's comments about how he wasn't her Daddy had finally hit its mark, and she brushed him off. In her head, she still called him Daddy, but out loud, she called him Sokka, refusing to add the Uncle no matter what. He wasn't her uncle, but he wasn't her daddy either anymore. He began to visit less, only seeing them when he was at Uncle Aang and Aunt Katara's island and her and Mama were too and she knew that her Mama was sad.
It was many months later that he showed up on their doorstep again; rain was pouring down and he didn't have a coat. Her mother scolded him but he didn't seem to hear. When her mother demanded to know what was wrong, all he could say was "Suki…the baby…Oh, Spirits, Toph…" Her mother seemed to understand what Lin could not, and her mother shooed her off to bed while she tended to Sokka.
A few days later, Lin attended her first funeral, standing beside her mother, who was supporting the grieving Sokka. She watched as a lady wearing lots of make-up and a dress was lowered into the earth in a traditional Earth Kingdom burial. The baby that Sokka had been expecting had died with its mother.
After that, Lin began to see him more frequently. Her mother insisted he move in with either them or Aunt Katara instead of living by himself, and to Lin's secret delight, he'd chosen them. He was there when she learned that she could earthbend, and he was there when Bumi tugged on her hair after she made him trip when he teased her. It was a whole year before she felt secure enough to attempt calling him Daddy again, and the tears in his eyes had almost made her change her mind. When he hugged her tightly, however, she relaxed happily.
Bumi continued to tease her incessantly, and she began to try to irritate him as well. When she was five and he was nine, their real rivalry started and escalated throughout the year. He had begun to cling to her Daddy more and more, and though Daddy had explained that it was because he could not bend, Lin had grown more and more jealous. They began to fight any time they were in the same room, and his pranks shifted focus so that rather than everyone getting pranked, with a slight focus on Tenzin, she was the only one who was pranked time and time again.
Finally, it seemed as though everyone had had enough of their bickering. It was a month after her sixth birthday, and her mother had come into her room early in the morning to demand she pack a backpack of clothes and come outside. She'd stumbled through the task before heading outside. Her mother had checked her pack and nodded her approval and then they were heading to the ferry. Daddy was waiting for them at the dock. At his side, to Lin's horror, was Bumi. She and Bumi exchanged mutual glares before obediently getting in the metal boat waiting, which was barely big enough for the four of them. Daddy began to row, and soon he was talking out loud about how to best steer the boat. She quickly lost interest, but Bumi looked like he was fascinated. By the end of the day, Daddy had switched places with Bumi, and Bumi was the one rowing the boat. It wasn't as smooth of a ride as when Daddy was rowing, but she had to admit that Bumi was doing well. When they docked at the end of the day, Daddy had set up a tent for he and Bumi while Mama had made rock tents for Lin and herself. Then Mama bent the boat into the ground for storage and Daddy told them that they'd be walking from here on out. Then Mama and Bumi had gone to gather wood for the fire while she and Daddy had begun to prepare dinner. Daddy showed them how to light a fire, and Lin fell into her tent when dinner was done, exhausted.
The trip that followed was one that Lin remembered with the deepest of fondness. She and Bumi had still argued, but the walking and tasks that her mother and father gave them left them with little time or energy to fight. Soon enough, the fights stopped altogether, becoming a waste of precious energy. Then her mother and father had begun setting them tasks to do together. At first, it had been tense, but they'd grown comfortable with each other in time. Almost as a reward, the exhausting tasks had stopped and her father had begun teaching Bumi the art of the sword. Lin would sit and watch from the sidelines, fascinated. Her mother would take the time to practice her own bending, of which she had taught Lin nothing, insisting that it wasn't yet time. She watched as her father showed Bumi the correct way to grip a sword, how to use it as an extension of himself, all with a stick rather than a real sword. Later that night, when everyone was asleep, she snuck off and found a stick of her own in the woods, attempting to copy what her father had taught Bumi. Each night, she did the same, until her father began to notice that she was losing sleep.
"Are you sleeping alright, Lin?" He'd asked. She assured him that she was asleep as soon as her head hit the ground, which she was. He seemed to believe her and she relaxed. Still, she was extra careful when she snuck off that night. Not as careful as her six-year-old brain would like her to believe, she later found, as her father snuck after her and watched as she practiced the latest moves she'd seen. He'd stepped out of the bushes, but she didn't see him until she pivoted during a move he'd taught Bumi and froze when she finally caught sight of him. She studied him for a moment, wondering if he was mad. He didn't look mad. But when he opened his mouth, she couldn't help but blurt out a "Please don't be mad". He'd looked faintly amused then and shook his head.
"I'm not mad." He'd said.
Afterwards, she'd joined he and Bumi in their daily sword lessons, with the only comment from her mother being that the footwork required for swordplay had better not interfere with her earthbending training later. Bumi had teased her at first, thinking that she would be behind, but her father's command for a quick bout to see what they both knew led to Bumi having both a greater respect for her and a stinging hand. She'd grinned and taunted him, which had led to a lecture from her father about being a good winner as well as a good loser and then challenging her to a bout himself. She'd quickly regretted her comments, though she could tell her father was taking it easy on her.
When they'd finally reached their destination, she was thrilled to learn that she would finally be learning earthbending. She could tell the Bumi was disappointed that she wouldn't be at sword lessons anymore but she was determined to do both. They were both surprised, then, when her father handed his sword to Bumi, and a spare sword from his pack to Lin before beginning to set up camp. Before either of them could begin to help, her mother was ushering them off. She led them to a set of caves, and then inside. When they were deep enough in, her mother informed them that they were to find their own way out, and that the cave walls moved, and that she was to learn earthbending in the process. She'd demanded to know how she would do that, and her mother had cryptically informed her that there were earthbending teachers in the caverns before disappearing into the earth below.
While Bumi and Lin had come to a tentative truce before, their experience in those caves was what started, and strengthened, their friendship. They bickered and fought like they hadn't since leaving Republic City, they found out who, or rather what, her mother meant by earthbending teachers, and they saved each other's life at least once apiece. When they finally exited the caves, Lin having earthbent them out, a bond had been forged between them that would not be broken by anything.
Which was why Lin blamed her parents for her friendship with the wild child of Avatar Aang.