Kaia Gerber is "down on [her] knees" with grief for her late brother Presley Gerber.

The 25-year-old model paid a heartfelt tribute to her sibling after he passed away at the age of 27 on 20 September, in which she remembered him as her "ultimate teacher" and the "love of [her] life" as she expressed her regret that he had passed away "first" after she had spent so many years following in his footsteps.

Alongside a throwback photo and a video of them dancing as children, Kaia wrote in a series of slides on Instagram: "Prez, I don't have the words, and maybe the tiny mercy is that we didn't need them all along. Words fail and words subtract and words summarize and words always lack truth. And how naive of me to believe that language could transcend grief.

"I've read all of the books and I have none of the answers. My articulation is no match against this, the magnitude of losing you. I am, quite literally, down on my f****** knees.

"You know how much I loved being your sister. It has always been, and will forever be, my favorite thing about myself.

"I came into this world with a white-knuckle grip around you. I had more love for you than I knew what to do with. and now that it has nowhere obvious to go, I'm certain it's going to swallow me whole.

"You believed -- always believed -- in people. You met cruelty not with anger, but with confusion. And curiosity. You believed people were inherently good because, Presley, you are inherently good.

"And somehow, through all of it, I got to be your sister, your lucky eternal witness, watching you do the most hard-core, heavy metal thing anyone can do: you stayed soft.

"You were my ultimate teacher while you were here and now I'm left wondering what I could possibly be taught in your absence, and I keep thinking about how when we were little, I got scared before getting my ears pierced, so you went first. You always went first.

"But when I held the earring they took from your body, all I could think was: that belongs to you. And I really f****** wish you hadn't gone first this time.

"To say loving you was the easiest thing in the world would imply that it required any effort at all. I was born loving you. I will die loving you. There was never anything else. I am learning how to love you when I cannot see you. How to tell you something without picking up my phone to call you. How to be your sister in a world that I can't reach for you in. I don't know how to do that yet."

The model and actress insisted she would "choose" to go through her grief again if it was the price she paid for their many years together.

She wrote: "You missed our date. I forgive you. Keep a place for me in the meantime.

"Because, Presley, even knowing where it ends, I would choose it all over again. Every second. I would walk knowingly toward this grief if it meant that on the other side of it I got to be your sister.

"Richard Fennyman once wrote to his wife after she died, 'You,dead, are so much better than anyone else alive. My darling wife I do adore you. I love my wife. My wife is dead.'

"Give me the whole impossible bargain. Give me every year with you and then give me this. Alright. I'll take it.

"My darling brother, I do adore you. I love my brother. My brother is dead. And this party sucks so bad without you, Prez.

"My best buddy, my great protector, the hardest I've ever laughed and the one true love of my life -- you lit all of my candles and took all of my monsters away. And I love you. And I love you. And I love you. Only you know how much. Thank you for being here."

She captioned her post: "i love you the bluest, to the moon and back and back again, forever big brother.(sic)"