What happens in the state/non-state of Nibbāna?

When a ‘person’ walks the path and reaches the goal of Nibbāna, ‘who’ actually reaches it? ‘Who’ walked the path and where did 'he' go when Nibbāna was reached, to be more exact? We as unenlightened beings see an arahant from the ‘outside’, as it were. Maybe we find him to be a holy, compassionate and wise being, but what is it like to be an arahant from ‘inside’? What is the nature of his personal being?