How to Character Climb using Spline?

Hi.
Now I’m making Paper2D game like ‘Ori and the blind forest’.
In "Ori and the blind forest’, Character can Stick to the wall and he can climb up&down.
I wanna make it.

Now I made it with Lock Character movement to Z axis.
He can move up&down Just straight along Z axis.

But the wall Can’t be always Just Straight line. ‘Wall’ is natural trees and rocks.
So I tried to use Spline.
Make Spline to follow surface nicely and make Chracter to move along Spline.

But there is problem. I can make the Character stick to middle of Spline.
Character can be Jump to wall. The point of Character holding the wall, can’t be always same.
But I can’t find the Node or some tutorials to get distance of spline compare with something else.

The Question : I want to Distance of Spline When Character try to holding it.