I want to know everything (that) there is to know about you.
I chanced to come across ‘expletive there’ in a syntactic textbook. It says in this type of sentences: there is an expletive, to know about you is the argument for the verb. Then [there is to know about you] is a complete sentence and there is no grammatical place for ‘(that).’
Then, ‘(that) there is to know about you ‘ is not a relative clause but complement for the noun everything (adjective clause), and so ‘(that)’ is not a relative pronoun but a conjunction, I think. Can this be a possible and appropriate thought?