Q:

Create a Scriptable Object Instance through a Constructor

I have a Character Scriptable Object and I have several methods in it. Like GetDate,GetAge (which I am not showing here). I have created a Constructor there, but I don't know how to create an instance of that ScriptableObject through the Constructor. That's what I want to do.
I have tried to just type the values in the inspector but that did not run the methods in the Constructor.
Character.cs
[CreateAssetMenu]
[Serializable]
public class Character : Scriptable Object
{
    // Attributes
    public string firstName;
    public string middleName;
    public string lastName;
    public string fullName;
    public bool isMale;

    // Constructor
    public Character(string _firstName, string _middleName, string _lastName, bool _isMale)
    {

        firstName = _firstName;
        middleName = _middleName;
        lastName = _lastName;

        // All The Functions 
        fullName = string.Format("{0} {1} {2}", firstName, middleName, 
        lastName);

        isMale = _isMale;

        // and all the other functions like date calculator.
    }
}

I want to create an instance of this Scriptable Object through this Construction. Something like
Character char("x", "y", "z", true);

or when I type the required fields in the Unity Inspector, then it will run all the methods like that full name string.format.

A:

You can't create a new ScriptableObject Instance using a constructor (much like a monobehaviour). 
so you need to use Unity API methods
I have added a working sample
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System;

[CreateAssetMenu]
[Serializable]
public class Character : ScriptableObject
{
// Attributes
public string firstName;
public string middleName;
public string lastName;
public string fullName;
public bool isMale;

// Constructor
public Character(string _firstName, string _middleName, string _lastName, bool _isMale)
{

    firstName = _firstName;
    middleName = _middleName;
    lastName = _lastName;

    // All The Functions 
    fullName = string.Format("{0} {1} {2}", firstName, middleName,
    lastName);

    isMale = _isMale;

    // and all the other functions like date calculator.
}

public void Initialize(string _firstName, string _middleName, string _lastName, bool _isMale)
{
    firstName = _firstName;
    middleName = _middleName;
    lastName = _lastName;

    // All The Functions 
    fullName = string.Format("{0} {1} {2}", firstName, middleName,
    lastName);

    isMale = _isMale;
}

}
and here is the class that creates the Character
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

public class Tester : MonoBehaviour
{
int characterCounter = 0;

// Update is called once per frame
void Update()
{
    if(Input.GetMouseButtonDown(0))
    {
        Character c = CreateMyAsset();
        c.Initialize("a", "b", "c", true);
    }
}

public Character CreateMyAsset()
{
    Character asset = ScriptableObject.CreateInstance<Character>();

    string assetName = "Assets/c" + characterCounter + ".asset";
    characterCounter++;

    AssetDatabase.CreateAsset(asset, assetName);
    AssetDatabase.SaveAssets();

    EditorUtility.FocusProjectWindow();

    Selection.activeObject = asset;
    return asset;
}

}

A:

No, not like that. It is still unclear why you have to use ScriptableObjects
instead of having a simple class (we came from this question) where you would simply configure those Characters e.g. in the Inspector or use eg.
new Character("abc", "xyz", "kfk", true);

You could however have some kind of factory method like e.g.
[CreateAssetMenu]
[Serializable]
public class Character : Scriptable Object
{
    // Attributes
    public string firstName;
    public string middleName;
    public string lastName;
    public string fullName;
    public bool isMale;

    // Constructor
    private void Init(string _firstName, string _middleName, string _lastName, bool _isMale)
    {
        firstName = _firstName;
        middleName = _middleName;
        lastName = _lastName;

        // All The Functions 
        fullName = string.Format("{0} {1} {2}", firstName, middleName, lastName);

        isMale = _isMale;

        // and all the other functions like date calculator.
    }

    public static Character CreateCharacter(string firstName, string middleName, string lastName, bool isMale)
    {
        var character = ScriptableObject.CreateInstance<Character>();

        character.Init(firstName, middleName, lastName, isMale);
    }
}

or alternatively do it e.g. in
private void OnEnable()
{
    fullName = $"{firstName} {middleName} {lastName}";
}

so it is there when the game starts.

If you really want to have it while typing those names in the Inspector you won't come around using a Custom Inspector by implementing Editor
#if UNITY_EDITOR
    using UnityEditor;
#endif

    public class Character : Scriptable Object
    {
        ...
    }

#if UNITY_EDITOR

    [CustomEditor(typeof(Character))]
    public class CharacterEditor : Editor
    {
        private SerializedProperty firstName;
        private SerializedProperty middleName;
        private SerializedProperty lastName;
        private SerializedProperty fullName;

        // called once when the Object of this type gains focus and is shown in the Inspector
        private void OnEnable()
        {
            firstName = serializedObject.FindProperty("firstName");
            middleName= serializedObject.FindProperty("middleName");
            lastName= serializedObject.FindProperty("lastName");
            fullName= serializedObject.FindProperty("fullName");
        }

        // kind of like the Update method of the Inspector
        public override void OnInspectorGUI()
        {
            // draw the default inspector
            base.OnInspectorGUI();

            serializedObject.Update();

            fullName.stringValue = $"{firstName.stringValue} {middleName.stringValue} {lastName.stringValue}";

            serializedObject.ApplyModifiedProperties();
        }
    }

#endif

alternatively to using #if UNITY_EDITOR you can ofcourse have the CharacterEditor in a complete separate script and put that one in a folder named Editor. In both cases the according code is stripped of in a build.

