Overall phrase structure
I'm not certain, but I suspect you might be misparsing the source.
どの人物もどこにでもいる普通の人達ばかりで
Your formatting suggests that you are considering the bolded part as a single piece. As my wife often says, "I reserve the right to be wrong" , and that said, here's how I would break this down:
[[どの人物]{Part A: Subject}] [も]{Inclusive} [[どこにでもいる普通の人達]{Part B: Predicate}] [で]{Copula}
↓
[[which person/character]{Part A: Subject}] [even]{Inclusive} [[regular people that]{Part B: Predicate} are anywhere] [is]{Copula}
↓
Anybody / any character [in the story] is just a regular person that you'd see anywhere...
The specific pieces
どのXも
This literally breaks down to [どの]{which}X[も]{even} → "whichever X", "any X".
どこにでも
This literally breaks down to [どこ]{where} [に]{in / to} [でも]{even, also} → "wherever [it] [VERB]s [in/to]", "no matter where [it] [VERB]s [in/to]" (whether or not to use "in" or "to" or nothing in the English depends on the verb).
Note that the でも is basically the same emphasizer you get in expressions like どこでも or だれでも. Due to the syntactic ordering of particles, どこでも + に (used here because of the verb) becomes どこにでも: the に gets inserted in the middle. We can't say *どこでもに.
This latter expression may change nuance slightly depending on the subject and verb used. In your sample phrase, it's いる, so "is" works.
We could also say things like どこにでも行【い】く, "no matter where [someone] goes" or to change it up a bit, だれにでも送【おく】る, "no matter who [someone] sends [something] to", or どれにでも偏【かたよ】る, "no matter which [one] [someone] leans towards / is biased towards".
Just remember that the に here is used because of the verb: いる in your example, 行【い】く and 送【おく】る and 偏【かたよ】る in my examples, all require that に.
