By Animation And Animator controller.

By Scripting.

First set up the scene like the given image.

Don't forget to add sprites folder that will be use for the the different animation to your scene, if you don't have the Sprites you can download it from here.

Create a 2D sprite object and assign the first sprite of the idle Animation to it. (Idle animation is the animation in which the character is just standing still and breathing no other activity).

Create the Script named AnimationWithScripting and attach it to hero Object.

Now, follow the below steps to create the different animation.
 

First set up the scene like the given image.

Don't forget to add sprites folder that will be use for the the different animation to your scene, if you don't have the Sprites you can download it from here.

Create a 2D sprite object and assign the first sprite of the idle Animation

Create the Script named as AnimationWithScripting and attach it to hero Object and write the following code in it

AnimationWithScripting.cs

using UnityEngine;
using System.Collections;
public class AnimationWithScripting : MonoBehaviour
{
    public SpriteRenderer spriteRenderer;
    public Sprite[] walk;
    public Sprite[] idle;
    public Sprite[] kick;
    void Start()
    {
        StartCoroutine(Idle());
    }
    void Update()
    {
        if (Input.GetKeyDown(KeyCode.I))
        {
            StopAllCoroutines();
            StartCoroutine(Idle());
        }
        if (Input.GetKeyDown(KeyCode.K))
        {
            StopAllCoroutines();
            StartCoroutine(Kick());
        }
        if (Input.GetKeyDown(KeyCode.W))
      {
            StopAllCoroutines();
            StartCoroutine(Walk());
        }
    }
    IEnumerator Idle()
    {
        int i;
        i = 0;
        while (i < idle.Length)
        {
            spriteRenderer.sprite = idle[i];
            i++;
            yield return new WaitForSeconds(0.07f);
            yield return 0;
                
        }
        StartCoroutine(Idle());
    }
    IEnumerator Walk()
    {
        int i;
        i = 0;
        while (i < walk.Length)
        {
            spriteRenderer.sprite = walk[i];
            i++;
            yield return new WaitForSeconds(0.07f);
            yield return 0;
        }
        StartCoroutine(Walk());
    }

    IEnumerator Kick()
    {
        int i;
        i = 0;
        while (i < kick.Length)
        {
            spriteRenderer.sprite = kick[i];
            i++;
            yield return new WaitForSeconds(0.07f);
            yield return 0;

        }
        StartCoroutine(Kick());
    }
}

Now go to Unity and add the sprites of walking, kicking and idle state in their following lists in the inspector as per the image.