Added: Add(), Remove() and CalculateFinalValue()

CharacterStat.cs

public float Value { get { return CalculateFinalValue(); } }

public void AddModifier(StatModifier mod)
{
    statModifiers.Add(mod);
}

public bool RemoveModifier(StatModifier mod)
{
    return statModifiers.Remove(mod);
}

private float CalculateFinalValue()
{
    float finalValue = BaseValue;

    for (int i = 0; i < statModifiers.Count; i++)
    {
        finalValue += statModifiers[i].Value;
    }
    // Rounding gets around float calculation errors (like getting 12.0001f, instead of 12f)
    // 4 digits is usually precise enough, but feel free to change this to fit your needs
    return (float)Math.Round(finalValue, 4);
}

using.cs

// We also need to add this outside the class curly braces
// to be able to use the Math.Round method
using System;