Bonjour Madame Aurélie ! Quelle est la personne que tu admires le plus ? Should not it be “la”?

Hi Varsha, 

In this case the superlative "le plus" is modifying the verb "admirer", and adverbs have no gender so they always use the masculine form:

Le plus/Le moins = The most/the least (Superlative with Adverbs in French)

However, if it were modifying a noun then you would agree with the gender:

Le/la/les plus and Le/la/les moins = The most and the least (Superlative with Adjectives in French)

Hope that helps!