C++ : A is not base class B

I'm stuck since many hours on an inheritance trouble and I need your light to unlock me...

My goal is to make a game, and I'm trying to create monsters.

error: ‘FastMob’ was not declared in this scope static_assert(std::is_base_of::value, "Mob is not a base class of FastMob");

FastMob inherit from Mob which inherit from Character, here is my .hh and .cpp

Mob.hh

#ifndef MOB_hh
#define MOB_hh

#include "Character.hh"

class Mob : public Character
{
private:

    //position
    double _x, _y;

    int _life;

public:
    Mob() {};
    virtual ~Mob() = 0;

    double GetX() const;
    double GetY() const;
    virtual int GetLife() const;

    void SetX(double);
    void SetY(double);
    void SetLife(int);

    void Draw() 
    {
    Character::Draw();
    }

    void Update() 
    {
    Character::Update();
    }
};
#endif

FastMob.hh

#ifndef FASTMOB_HH
#define FASTMOB_HH

#include "Mob.hh"

class FastMob : public Mob
{
public:
    FastMob();
    ~FastMob();

    void Update() override;
};

#endif

FastMob.cpp

#include "FastMob.hh"
#include "Timer.hh"

FastMob::FastMob()
{
    Mob::SetLife(25);
}

void FastMob::Update()
{
 // some code
}

FastMob::~FastMob()
{}

Character.hh

#ifndef CHARACTER_hh
#define CHARACTER_hh

class Character
{
    public:
        Character() {};
        virtual ~Character() = 0;

        virtual void Update() = 0;
        virtual void Draw() = 0;
};

#endif

Thanks !