If the antecedent of 'who' is 'people,' a plural noun, then it would take a plural verb, in this case, 'believe.' If the antecedent of 'who' is 'Jasper White,' a singular noun, then it would take a singular verb, in this case, 'believes.' Since 'who' can be both singular and plural with no change in form, either verb can be correct.
The only clue to favor one interpretation over the other is the absence of a comma after 'people.' Strictly speaking, a comma in that location would make the antecedent 'Jasper White.' However, the absence of a comma in that location does not necessarily indicate that the antecedent must be 'people.'
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