(ABC News) -- Nicole and Jonas Maines aren't your average siblings. Though they may not look like it, these 18-year-olds are actually identical twins.
While they share the same genes, their genders are different. Nicole recently made the brave decision to undergo sex reassignment surgery.
"Being a girl just felt right," Nicole continued. "It felt like that's what I was supposed to be, and I've never looked back. I've never felt like I made the wrong decision. I've never felt like I'm doing the wrong thing with my life. I just knew this is exactly where I'm supposed to be right now."
Like her brother Jonas, Nicole was born as a boy and was named Wyatt.
"I knew that I was trans when I was 3 years old," she said. "Well, I didn't know trans because I didn't know there was a word for it, but I just knew that in my head and my heart that I was supposed to be a girl."
For her parents Kelly and Wayne Maines, it was puzzling at first.
"I knew that Nicole was different. I didn't know what it was. I didn't know that she was transgender at 3, I just knew she wasn't like the other twin," said Kelly Maines. "She played all the girl roles, she always wanted to dress as a girl character, so I knew there was something different about her."
"I just made it a point of making sure that I could do whatever I could do for her to get her where she needed to be and also to know she always had a safe place to be with me," she added.
Nicole's father Wayne struggled with the idea of letting go of one of his sons.
"When the twins were born I had these dreams," he said. "I already knew what deer rifles I was going to buy, and by the time they were 2 years old they were going to have them. Football, basketball, everything we thought about for me was 'the boys.'"
He tried ignoring Wyatt's desire to be different, but over time he came to realize that this is what Nicole wanted.
"When we let her go out in a dress, and this kid beamed and was happier," Wayne Maines said. "Then I started to think about it, 'man, I gotta change who I am.' … I had to dig deep into my soul and say, 'Hey what are you afraid of?' …You gotta dig deep and ask yourself, 'How much do you love your kid?' and do the right thing."
So Kelly and Wayne supported Wyatt in having long hair and wearing girl's clothing, and Nicole wasn't shy about telling people who she was. When she was in first grade, Nicole said she would go up to someone and say she was "a boy who wants to be a girl."
"I didn't think there was anything wrong with it," Nicole said. "And of course, kids' parents were like freaking out and kids were like 'Oh cool, I like trucks.' … it was just a thing that came with my introduction."
By the time she was in fourth grade, Nicole began to think about changing her name from Wyatt to something new she felt would be more of "a girl's name." She started looking at TV characters that resonated with her and settled on a character's name from one of her favorites.
"There was this character on a show called Zoey 101," she said. "It had Jamie Lynn Spears, I think her name is and like one of her classmates' name was Nicole. And she was just like really quirky and like fun. And I was like, 'oh she's a lot like me. I'm a Nicole.'"
By age 8, "Wyatt" was no longer, but by middle school, Nicole said she was beginning to feel ostracized by her classmates.
"Kids would say there's something wrong with me," she said. "And I would come home crying on the bus like 'Mom, Dad I'm a freak. There's something wrong with me. This isn't right.' I'm like, 'It feels right to me but it can't be right because people are telling me that it's not right.' And [my parents] would tell me, 'Listen, don't listen to them. Be true to yourself. This is who you are. Just be true to you.'"
Jonas was fiercely supportive of his sister during this time.
"When I was a kid my dad told me to look after her and to protect her and do what you have to do," he said. "So that was a huge responsibility that I had to carry around for a lot of years."
Then there was the school bathroom issue. For most of her time in elementary school, Nicole used the girl's restroom without issue, until a grandfather of one of her classmates complained to school officials.
The Maines family filed and won a discrimination lawsuit against their school district in Maine, scoring a big human rights victory for trans kids. The case changed Maine state law.
"It really did just start out with me calling the Maine human rights commission and going, 'Hey - this is so not right' and I never realized that it would ever spiral to where it did," Kelly Maines said.
Since the lawsuit, Nicole has been in the media spotlight and her family has now become activists for the transgender cause. They are the subject of a new book called, "Becoming Nicole: The Transformation of an American Family," to be released on Oct. 20.
It's been a pivotal year for the transgender movement, in part because of Caitlyn Jenner's coming out story. Her speech at the Espy Awards, in particular when Jenner talked about trans youth, struck a chord with Nicole.
"I'm like, absolutely, because trans youth shouldn't have to take it, they should be able to go to school," Nicole said. "Trans kids, just kids in general, are not to be looked upon by society through a magnifying glass."
Nicole is part of a new generation of out and proud trans-kids, like Jazz Jennings, the star of the TLC show, "I Am Jazz."
Despite increased social acceptance, the transgender movement isn't without controversy, especially when it comes to teenagers undergoing the life-altering sex reassignment surgery.
Just a few days before her scheduled procedure, Nicole talked about her decision to go through with the surgery and called it her "light at the end of the tunnel."
"This is always what I needed," she said. "Over almost 18 years of life I've never thought back of you know what no I don't want surgery… I just felt like this is that final piece of the puzzle where my body is finally going to match what's happening in my head and my heart."
Now Nicole and her brother Jonas are in the middle of their first semester of college. It will be the first time the twins will be on their own. These days, the most pressing thing on Nicole's mind is passing midterms.
"I'm happy," she said. "I like it. I mean I got my hair done today, I got my make-up on. I'm happy! I mean, I'm Nicole. I'm incredibly happy."
