So according to allkpop.com

Singer IU revealed where she received inspiration to write her self-composed song, “Peach“.

When questioned about her self-composed song “Peach”, she confessed, “I wrote the lyrics to “Peach” while thinking of f(x)‘s Sulli from a male perspective.” The singer didn’t go into further explanation as to why she thought of Sulli, but the lyrics do give a good visualization of a man falling in love with a beautiful girl at first sight.

Sulli even sang "Peach" for her drama, "To The Beautfiul You".

The lyrics goes...

My eyes keep going to that white face
Why don’t I even get sick of you?
When you slightly smile at me, I really go crazy
How can you be so pretty baby?

How can I explain this feeling?
When I see you, my heart becomes numb and sore

Oh, with what word can I explain you?
All the words of the world is probably not enough

With those legs that are so pretty by just standing still
You walk toward me and you hug me

You know he’s so beautiful
Maybe you will never know
I want to hide you in my embrace

I’m not saying this out of a young heart
But I really want to marry you

Oh, with what word can I explain you?
All the words of the world is probably not enough

With those legs that are so pretty by just standing still
You walk toward me and you hug me

Telling you multiple times is not enough
With this tickling voice
that only knows you
I will sing for you

My heart keeps going to you
I’m really going crazy

I think IU likes Sulli, what do you think? kekeke~ me & my imaginations. O well! The story goes...