I need to access the R15 parts of a character to adjust its properties how would I do this?

You use a for loop on your character and set what you want returned, which in this case is the BasePart.

local bounkey1 = "f"

local usingPower = game.ServerStorage.powerValues.usingValues

local xp = game.ServerStorage.powerValues.XP

local Player = game.Players.LocalPlayer

local UIS = game:GetService("UserInputService")

    UIS.InputBegan:connect(function(InputObject,Processed)

      if not Processed and usingPower == false then

          if InputObject.KeyCode == Enum.KeyCode.boundkey1 then

              usingPower = true

               powerXP = XP - 40      
				for _, v in pairs (Player.Character:GetChildren()) do
					if v:IsA("BasePart") then 
						v.Transparency = .5 -- Change here
					end
				end         

        end

    end

end)