Invader is a widely known French urban artist in real life. He is known for his anonymity, and has never been spotted in the flesh, although his art work is exceedingly popular. He is highly praised for his graffiti work of alien beings and pop culture references (including renown games such as Space Invaders and Pac-Man).
Players have theorized that he may of been included in the Grand Theft Auto series due to multiple pieces of graffiti resembling his artworks. In the tunnels and alleyways of Grand Theft Auto IV, multiple drawings are presumed to be his art work. Some notable artworks include an alien head, and a sketch of the video game character Pac-Man. As this is his main style of work, fans have put forth theories that these two artworks in particular point towards the existence of Invader in the game.
There is also a collectible in Grand Theft Auto V, which is a reference too. There is also a hooded pedestrian wearing a monkey mask, who is seen doing making the pieces of graffiti.