"This person has been my stepdad for half my life," she said. "But life is just about being happy -- that's the one common denominator that everyone can understand. If you aren't able to be you, who can you be?"
On "From The Desk Of," a section of Kim's new app, she opened up about Caitlyn and those much-discussed rumors that, when she was young, she saw her stepdad dressed as a woman. In a strange way, she was sort of prepared for it because her father, Robert Kardashian, had mentioned it before.
"Had I not overheard my dad and his friends discussing rumors that Bruce would dress up as a woman years before," she said, "I never would have believed what I was seeing. I thought my dad was jealous of my mom's new husband, that he was joking with his friends on the phone, but suddenly I knew it was all true."
Kim held tightly to her stepdad's secret for years, only telling older sister Kourtney.
"We did not tell my mom, either," she said, saying she was about 11 years old when she first heard the whispers of Bruce's secret. "I'm not sure why I didn't tell my mom. I guess I thought it would break up her marriage, and Kendall and Kylie were so young ... I just felt so bad."
It wasn't until she was 30 years old that she and Caitlyn, then living as Bruce, spoke that day when she saw him donning women's clothing.
"He explained to me that he always felt he was trapped in the wrong body. He said he was going to therapy and had dealt with it…" she said, "I never really thought he would one day transition -- that was never discussed. So I never could have foreseen that, in the future, Bruce would transition into Caitlyn."
The meeting actually gave her a better of understudying of why Bruce was the way he was.
"I walked away from this talk maybe understanding Bruce a little bit better as to why he did a lot of solo sports and spent a lot of time alone, because he didn't really connect with many people," she said, "but I never walked away thinking he was a different gender. Maybe it wasn't explained well enough to me or maybe Bruce still wasn't sure and didn't know how to communicate what he felt."
Kim said she "started to freak out" when she started seeing pictures of Bruce with his nails painted, but he eventually decided he wanted to live his like publicly as a woman. "This was such a huge weight off of my shoulders, that Bruce can live his authentic self and become Caitlyn," she said.
Now, Kim and Caitlyn's relationship has never been better.
"I'm just so proud of Caitlyn, and our relationship is even stronger now," she said. "And, after all, she asks me for style and glam tips all the time. What's more fun than that?"