2.5d controller

Hello,
I am making a 2.5 d game , like with camera view on 2d and with 3d models (just explaining for some reasons ,i know you guys know about 2.5d) , Just have a problem about controlling ,i don't want to my character to go to x axis (in 2d i know it's x and y) in 2.5 d i have to lock z axis , but my player is 3d and it is looking at z so i need my player to go forward with it's look and i want to lock x axis so my player smoothly move to y(up/down) and z(forward/backward),I tried that -

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class force : MonoBehaviour {
5.  
6.     // Use this for initialization
7.     void Start () {
8.  
9.     }
10.    
11.     // Update is called once per frame
12.     void Update () {
13.         Time.timeScale = 0.5f;
14.         if(Input.GetKey(KeyCode.UpArrow))
15.            {
16.             //GetComponent<Rigidbody>().velocity = new Vector2(0, 12);
17.             GetComponent<Rigidbody>().AddForce(Vector3.up * 200,ForceMode.Acceleration);
18.         }
19.         if(Input.GetKey(KeyCode.DownArrow))
20.         {
21.             //GetComponent<Rigidbody>().velocity = new Vector2(0,-12);
22.             GetComponent<Rigidbody>().AddForce(Vector3.down * 200,ForceMode.Acceleration);
23.         }
24.         if(Input.GetKey(KeyCode.LeftArrow))
25.         {
26.             //GetComponent<Rigidbody>().velocity = new Vector2(-12, 0);
27.             GetComponent<Rigidbody>().AddForce(Vector3.back * 200,ForceMode.Acceleration);
28.         }
29.         if(Input.GetKey(KeyCode.RightArrow))
30.         {
31.             //GetComponent<Rigidbody>().velocity = new Vector2(12, 0);
32.             GetComponent<Rigidbody>().AddForce(Vector3.forward * 200,ForceMode.Acceleration);
33.         }
34.         }
35. }
36.  

but it won't work at all , even if i tried to lock z with vector 2 but it still move to z but i want to lock x , and i tired vector 3 but it still won't work , it sill moving to x . any help, thanks .