How to make a script make a function appear only to the player who touched it?

This is simple and easy to do.

First create a local script, you can put this script inside anywhere that can work(I'll put it inside StarterPlayerScripts).

Inside the local script type:

local part -- location of your part in your game.
local transparent_part -- location of your part you want transparent

local Players = game:GetService('Players)
local plr = Players.LocalPlayer -- our client(player)

local transparency = 1 -- the number of transparency you want the part to get

part.Touched:Connect(function(hit)
 if hit.Parent:FindFirstChild('Humanoid') then -- verifies if hit's parent has 'Humanoid' as a children.
   if Players:GetPlayerFromCharacter(hit.Parent) == plr then -- verifies if the player that touched the part is our client.
      transparent_part.Transparency = transparency -- sets transparency to part
   end
 end
end)

local Players = game:GetService("Players")

local Part1 = workspace.Part1
local Part2 = workspace.Part2

Part1.Touched:Connect(function(Hit)
    local Player = Players:GetPlayerFromCharacter(Hit.Parent)

    if Player then
        if Player ~= Players.LocalPlayer then
            -- Someone touched the brick, but not me!

            Part2.Transparency = 1
        end
    end
end)

It's pretty self explanatory, also this is still a LocalScript, I'm just kind of confused on your question