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literally means "in that point/respect" but here it means "Compared to that, ..." or "Contrary to that, ..." (≂ ). I feel like in this sense is used in casual speech rather than in formal speech/writing. And it's (not , etc.) when used in this sense.
_lit._ "Compared to _that_ (= ), minor characters are good."
"Compared to / Contrary to (being) a main character (who is busy), (being) a minor character is good (cos it's carefree / not difficult or busy.) / [Thanks to @Sjiveru] it's better to be a mob.