The usual way is that someone is intelligent (I mean within a population - I am not talking about the evolution of population differences) is by what is termed Good Genes: that is, having few faults or errors - the person has a normal brain but with nothing (or nothing much) wrong with it: in other words he has a low load of deleterious mutations (or, conversely, he is not suffering from mutation accumulation).
But there is another way - which is by having an Endogenous personality - which means that his brain is purposely designed (by group selection) to be creative, to make breakthroughs - he is, in sum, a genius (albeit very probably not a world historical genius; but a tribal or local genius). My assumption is that - if we could know this - we would see a brain wired-up to be intelligent.
Therefore the brain of an Endogenous personality has high intelligence not so much negatively from lack of mutations; as positively - because it is a brain specialized to be highly efficient for the purpose of creative discovery.
And this is why the genius has a special (Endogenous) personality. Usually personality and intelligence are almost distinct and little-correlated; but the brain of a genius is differently wired from a normal brain: it is a specialized and purposive brain, a lop-sided brain, a brain in which circuits usually used for social intelligence and reproductive success are co-opted to this purpose.
In sum, the brain of a genius is one that is specialized for creative discovery and both high intelligence and an 'inner-oriented' personality are features of this specialization.