Final answer:
The sentence provided does not contain an underlined pronoun; therefore, it is not possible to identify the case and function. Normally, pronoun cases can be subjective, objective, or possessive, depending on their role in the sentence.
Step-by-step explanation:
The case of the underlined pronoun in the sentence "A legendary equestrian vaulter is sitting behind you." needs to be determined. Since the sentence does not include an underlined pronoun, it is not possible to determine its case or function. However, if one of the words were to be a pronoun, we would then define its case based on its grammatical role in the sentence.
Pronoun case can be subjective, objective, or possessive. Subjective case pronouns are used as the subject of a verb, like 'he' in "He runs fast." Objective case pronouns serve as the object of a verb or a preposition, like 'him' in "Laura gave him the baseball."
Possessive case pronouns demonstrate ownership, like 'his' in "The book is his." Knowing the specific pronoun in question would be crucial to correctly determining the pronoun's case and function in the given sentence.