Calista Cod is the stogie chain-smoking, no-nonsense Caucasian mother of Emerson Cod. The exact nature, biological or legal, of their bond is never discussed in the show. She is also, like her son, a private investigator. The two have a complex personal dynamic, being in the business of secrets, but having officially sworn never to keep anything from one another.
She returns to visit her son in "Frescorts". At the end of the episode she speaks of "your [Emerson's] childhood with your crazy mother", and the pair agree to start afresh, building a parent-child dynamic, rather than the 'best buddies' thing they always had.
She has no further appearances in the series, but may well have been intended to recur in Season Three, had that materialized.