Sure, in the official LINE help and elsewhere, there’s a lot of advice about hiding, and deleting and blocking friends.
But none of that tells you how to get back to the exact same state you were in before you added them.
All the methods have some side effect leaving some permanent difference.
I want to get back to the exact same state that I was in before I accidentally added them. In other words I want their state in regards to me and my state in regards to them to be exactly like it was before I ever added them.
On Facebook, one could simply click Unfriend.
But in LINE it is much more complicated.