How can I make a player able to pass through a walking NPC?

Solution

The easiest and most efficient way to achieve this would be using collision groups.

Physics service

Most things involving collision groups use the Physics service, which can create collision groups and disable/enable collision between collision groups.

For the purpose of disabling NPC to NPC collision, the :CreateCollisionGroup() function and the :CollisionGroupSetCollidable() function can be used. The former creates a collision group with a given name and the latter enables/disables collisions between two groups or members of the same group

Application

With all said, time to put the functions into effect :3

step 1: reference the physics service

step 2: create an NPC collision group

step 3: disable collisions for members of that group

step 4: add all bodyparts of the NPCs to the NPC collision group

local PS = game:GetService("PhysicsService")--1
local NPCs = workspace.NPCs

local CollisionGroupName = "NPCs"

PS:CreateCollisionGroup(CollisionGroupName)--2
PS:CollisionGroupSetCollidable(CollisionGroupName, CollisionGroupName, false)--3

local function SetCGRecursive (object)
    if object:IsA("BasePart") then
        PS:SetPartCollisionGroup(object, CollisionGroupName)--4
    end
    for _,child in next, object:GetChildren() do
        SetCGRecursive(child)
    end
end

for _,NPC in pairs(NPCs:GetChildren()) do
    SetCGRecursive(NPC)
end

Additional things

Also, as you are making an RPG which probably involves combat, you can use the ChildAdded function to see when something has been parented to another object

Ex:

NPCs.ChildAdded:Connect(function(child)
    SetCGRecursive(child)
end)

A couple of reference links:

tutorial for disabling player-player collisions

Physics Service

Generic for loops

Generic for loops 2

recursive functions

child added

Hopefully this helped and have a great day!