This “lo” has two usages, neither of which is a personal pronoun.
Lo mejor = le mieux = the best (that which is best): lo is behaving like an article;
Lo que no has visto = ce que tu n’as pas vu = what you haven’t seen: lo is behaving like a subject pronoun.
The other “lo” which is an object pronoun is different.
Is the mild gender distinction between inanimate and animate objects that English currently makes a case of things coming full circle? I read somewhere that PIE’s genders were originally used to distinguish between animate and inanimate before morphing into the more common masculine, feminine and neuter that we see in many modern languages today. Further, isn’t the inanimate/animate gender that we see in English quite rare amongst modern Indo-European languages?
(e.g. in English:
Is that your bookshelf? Yes, it’s my bookshelf.
Is that your dog? Yes, he’s my dog.)
The demostrative pronouns in Spanish are “est*, es*, aquel*”, where the * denotes gendered terminations - they do have all three genders; in class they’re usually recited as “esto, eso y aquello”, that is, in the neuter forms. The demostrative articles are quasi-identical but with only two genders (m/f): éste/a, ése/a, aquél/la.
And lo can, indeed and among other things all of which are neuter, be a personal pronoun, in the accusative. Or so RAE says.