Moses Johnson is basically just six and a half feet of muscle and guy with the scar as far as Shawn and Gus are concerned in "High Top Fade-Out". He is the terrifying looming presence dogging their steps as they try to unravel what it is that got Leonard "Diddle" Callahan killed, and he abducts them.
However, as ever, things are not what they seem, and when they come round in the SBPD the duo discover that this is, in fact, the detective who has been looking to smash a major narcotics operation which, it appears, Diddle stumbled across. The kidnapping was to prevent months of police work being blown by Shawn and Gus' antics. Now openly on the same side, the trio go together to arrest Chelsea, who was using an electronic 'back door' to feed information about where the police were storing drugs to a criminal cartel. Detective Johnson gets his culprit, but perhaps more importantly, she seems genuinely remorseful, and likely to provide him with the means to bring down the entire operation.
Detective Johnson is played by Tony Todd.