Mouse is shrewd, a kleptomaniac, and a compulsive liar. Growing up an orphan that never knew his parents, he earned money as a courier and thief, eventually leaving the city's corrupt thieves guild to create his own guild meant to serve as a safe haven and second family to streets kids like him who had nowhere else to go.

One day, he took a special job from a mysterious stranger to transport a sealed box across the city. Ignoring the stranger's instructions, Mouse opened the box and unleashed the terrifying monster within: A symbiote lifeform named Khoshekh, who promptly bonded with Mouse to use his body as a host. The following days were dark for Mouse, but over time he learned to communicate and negotiate with the dark being he now shared his body with. Regaining more and more control, he started to use the newfound powers of his alien-infused body for good to protect the members of his guild of street thieves.

Later on, Mouse's desire to make the world a better place for street children without a home led to him taking part in a grand effort to civilize and populate a wild region of the land. Together with many brave adventurers, he contributed to the founding and establishing of a new kingdom, in which he even eventually took a position to manage the kingdom's finances. Being a government official might seem like a very unfitting role for a street thief who'd always gotten into problems with authority, but Mouse had always been highly political and knew a great deal about the theories of social governing.

At first glance, one does not notice Mouse's body is host to a second, completely alien life form, although he can sometimes be caught seemingly talking to himself. He comes across as sarcastic and sharp, but this aggression serves mainly to cover his insecurities. He also has synesthesia, which allows him (among many other blessings and curses) to recognize people's voices very well, even if he just met them briefly long ago. His best friend is Moyo, a half-dog who likes to use Mouse as his personal chew toy.