Translation:It is character that makes the difference.
This is the first time I've come across this construction when "a" + infinitive is translated "that [present tense]". Why is the clause not introduced by "che", as I was growing comfortable expecting? (So, of course, I translated this, "He is the character to make the difference," but perhaps "carattere" doesn't have that dual sense in Italian?)
This actually answers the question I apparently had three years ago. That translation would only be possible if the Italian sentence used "che" and not "a".
Edit: Never mind. While the preposition choice might also be an issue, "carattere" is also the wrong word for the type of "character" you're referring to. That can only be "personnagio."