As the first title in what Ubisoft hope will be a successful 'Assassin's Creed'-esq franchise it's no surprise that the characters are painted with very broad strokes rather than nuance and subtlety.
Aiden Pearce is part rogue, part film-noire private detective. Seemingly callous at times, especially with regards to his family and more physically vulnerable allies, the mechanics of the game tend to dictate that he is an anti-hero of sorts. Although he shows himself as caring, primarily in conversation with family or Clara, both life and individual privacies are troublingly irrelevant both in terms of the ongoing narrative and the wider themes Watch_Dogs could bring to bear. As the Vigilante the public at wide seem to gawp and stare rather than react assertively, perhaps reflecting how minor a part such a character could play in their day-to-day lives.
Brenks is a loose cannon, driven almost beyond sanity to discover who took both his health and ill-gotten wealth. Belligerent and manipulative, he's swiftly set up as the chief antagonist of Watch_Dogs. Even given his unpleasantness it's difficult to sympathise with Aiden, who you want to succeed in this conflict purely because it might help keep innocents out of harms way rather than sympathy for either him or great antipathy for Brenks in his antagonism towards Aiden.
Clara Lille AKA BadBoy17 is a member of the DedSec anti-corporate hacking group and general rebel-seeking-a-cause eager to take down the Blume Corporation. Clara has largely replaced Brenks in Aiden's world, providing him with leads and the hacking tools to complete missions with the minimum of fuss. What isn't known is her underlying motivation, if she really is just against Blume or if there's something more personal at play.
Jordi Chin is Aiden's fixer, supplying the more... solid tools of the trade. He is Aiden's link to the seed(-ier) elements of Chicago's underworld, supplying cars, weapons and explosives to order as well as muscle when Aiden needs backup. Jordi's relationship with Aiden is complicated by to his psychopathic tendencies, for Jordi problem-solving usually begins and ends with at least one body. For his part Aiden resists in being drawn too far down Jordi's path.
The Blume Corporation are the designers and operators of ctOS for the city of Chicago, but rumours persist of an involvement in some of the shadier aspects of Chicago business. Their chief opposition is DedSec, an apparently loose coalition of hackers and anarchists thematically similar to Anonymous but with a willingness to stretch their reach into actively disrupting city operations. You will often hear the competing views of Blume and DedSec in news reports over the radio.
Other major characters also appear throughout the game but revealing them would venture too far into spoiler territory. Suffice it to say that these broad strokes are repeated throughout.
While the named characters do express unique and distinct personalities this isn't the case for the wider population of Chicago. The vast majority of your interactions are entirely mundane, generally relegated to getting a stern look from a passer-by as you bump into them or cries of alarm as you gas of your vehicle. Even without your input the NPCs and their cars wander aimlessly, with the occasional traffic accident or theft occurring if you are within earshot as a means of adding colour.
Depth is added to the world when you 'profile' them & 'hack' NPCs to discover unusual facts about them and gain access to their smartphone. Although this will generally yield only cash, it will sometimes reveal the location of a cache of goods hidden nearby or a crime about to be committed. Rarely, hacking phones will allow you to uncover 'fixer' opportunities or find gang hideouts, triggering sub-missions which grant experience and reputation. There's validation to this interaction however; you won't meet them again later with an updated backstory, they just blend right back into the crowd.
The hackneyed dialogue belies the voice acting quality from all quarters. Noam Jenkins in particular is excellent as Aiden Pearce, but throughout the actors have done sterling work giving life to what otherwise would have been pretty bland characters.
Audio Logs & Sub-Plot Elements
Much like BioShock and Deus Ex a large portion of the game outside of the main campaign revolves around collections of audio logs and completion of minor tasks. Within Watch_Dogs these titbits of information flesh out the social conflicts in near-future Chicago and teach you more about Blume, DedSec and ancillary characters important to the genesis of both organisations. Completing all these collections – a fairly trivial task that requires time rather than applied effort – will yield additional missions focussing on the game sub-plots.
Watch_Dogs also gets a point in the credit column for fully voice acting each of these logs, as well as including voiced telephone conversation intercepts rather than relying on transcripts/diaries as many other games have done in the past.