I there a word that best describes someone who forcefully describes or explains them self to others?
I believe the verb you need to include is expound. The noun expounder does exist, but it is unusual.
My own syntax would be something like: Jim expounds vehemently on why he thinks Britain should remain in the European Union.
Oxford Dictionary Online
expound (v) present and explain (a theory or an idea) in detail: (trans) he was expounding a powerful argument (intrans) he declined to expound on his decision.
By itself expound does not entirely get across the idea of forcefulness, (though it is stronger than explain)) and hence the need to add the adverb vehemently.