According to Caballero, Monster represents his father who he had "a difficult relationship with" and in order to tell the story of Monster, he had to include both the "positive aspects of the man I loved and the negative aspects of the man I feared."
For help in designing Monster, Caballero turned to artist Nilo Rodis who's previous endeavors include Pixar and Star Wars. The two met while both were at EA and Nilo was able to provide help in designing the characters in way that was both appealing to the gamer, yet still personal enough for Caballero to relate to.
In the end, the duo designed a Monster that made you "want to play with him [and] help him, even though he’s a beast who could fly into a rage and kill you."
The full blog post is through the links above.
More information on the game, which was announced in June last year, is expected soon.