This question resolves according to this source, or the first non-ad result on a Google search for "World's richest person", once an individual's publicly known net worth exceeds $1T USD.
@NickAllen as in "an AI" beats less-specific answers that end up being correct, like "a non-human intelligence".
Other than AI explosion or hyperinflation scenarios, I think that the first person to become a trillionaire in USD probably hasn't been born yet. Simply put, the closest anyone has gotten is still a rough order of magnitude away, and the time series for increasing absolute magnitude of the wealthiest single person on earth in USD doesn't suggest we're due for another power up anytime soon.
Thus, to get there in a short time frame, you either anticipate one person suddenly accumulating vastly more value than any previous person in history has done (in terms of rate), or you expect the real wealth denoted by the figure of a trillion to be vastly depreciated.
@AndrewHartman yup, that last possibility is the real curveball here.
So much so that the "who" depends mostly on the future value of the nominal dollar. Put another way, this is a concrete formulation of a hyperinflation prediction market.