The benoni (בינוני)—the intermediate person—is the central figure of the Tanya, and chapter twelve defines him precisely.
The benoni has never sinned. Not once. Not in action, not in speech, not in sustained thought. The three garments of his animal soul—its impure thoughts, words, and deeds—have never gained enough power to control his body. Only the three garments of the divine soul operate through his limbs, his mouth, his brain. He is, by any external measure, a perfectly righteous person.
But internally, the benoni is at war. The essence of his divine soul—its ten inner faculties—does not hold "undisputed sovereignty" over the small city of his body. During prayer, when the mind is focused on God, the divine soul rules. But outside those elevated moments, the animal soul reasserts itself. Desires return. Distractions crowd in. The benoni must fight them off, again and again, forever.
Rabbi Schneur Zalman draws the battle lines. During the Shema and the Amidah, the benoni binds his intellectual faculties to God and arouses burning love in the right side of his heart. In those moments, the evil in the left side of the heart is genuinely subdued. But afterward, the evil can reawaken. And so the benoni lives in a cycle: elevation during prayer, struggle during the rest of the day, elevation again, struggle again.
The crucial distinction: the benoni's evil nature is never destroyed. It is contained. The tzaddik (a righteous person) has transformed his evil into good. The benoni has only suppressed it. But here is the Tanya's great consolation—and the reason the book is subtitled "The Book of the Benonim." This level is achievable by every person. You do not need to transform your nature. You do not need to eliminate desire. You need only to win the battle, every single time it is fought, for the rest of your life. That is enough. That is the whole purpose of creation.