Lucas doesn't really remember his first dad anymore. He knows his first father drowned in the lake, dragged down by some vengeful spirit or something. None of the grownups ever felt the desire to explain, except for Dean, his second father.
He knows his mother would rather he not think of Dean like that. He also knows that his mother secretly wishes sometimes that Dean had been his first father—Dean wouldn't have drowned. Dean would have fought back. Dean could understand him when he wasn't talking. Dean is his second father.
Lucas's first father is indistinct in his mind. The school psychiatrist says he must have blocked him out when the trauma got too bad. Dean is his second father, the man he remembers after that. Dean could talk to him, and he would understand the feeling of fear that came with losing a parent. He even heard Dean's brother, Sam, say that Dean never talked about their mother. That made Lucas feel special.
His second dad called sometimes, not often, but every once in a while so that they knew he was okay. Dean must have asked his mom when Lucas' birthday was, because he always called then. Every few years he would stop into town too. He showed up for Christmas when Lucas was nine. No one could stop smiling the whole time. His mom made extra plates of cookies, and Dean stayed up late with him to play with the army men. He had to leave soon after, but they were happy to have him for any amount of time.
By the time he was thirteen, Lucas had finally learned what Dean meant when he taught him to say "Zeppelin rules!" He's pretty sure he was the only eighth grader to even know who they were. His mom bought him a record of them for his birthday, and he felt like he was closer to his second dad.
His second dad came home to stay when Lucas was sixteen. He was way more scarred now, with one big one stretching across his side, from his knee to his ribs. Lucas knew enough about anatomy to know that this was probably why Dean couldn't really walk well anymore. He needed a cane, and an occasional hand, which Lucas was happy to provide. Dean said that Sammy was safe, the demon was gone, but their dad was dead now too. He was too hurt to hunt anymore; he would have to retire. Lucas didn't mind that his dad had to stop hunting. It was nice to have him around all the time.
Lucas' mom was happier now too, her small face widening every time she saw Dean. He found a job with a local mechanic, and he even taught Lucas to drive the Impala, which Uncle Sam assured him over the phone was a rare honor. His dad refused to teach him to hunt, but he would teach him to fix cars, so every day after school he went to the garage and earned some pocket money.
Lucas can't remember his first father, because it's his second father who did everything that counts. His second father gave him "the Talk" when his mother was too embarrassed. His second father was the one sitting up at night to make sure he got home from his dates before curfew. His second father is the one who saved his life when he was four. Dean is his second dad, and the one that really matters.