Complex sentence: Despite initial doubts because of her uncanny likeness to his enemy, Gilbert ultimately became convinced she wasn't Lia, grew to trust her, slowly developed feelings for her, which ultimately stirred his heart to beat again, his powers temporarily weakened.
Split into simpler sentences: Despite initial doubts because of her uncanny likeness to his enemy, Gilbert ultimately became convinced she wasn't Lia and grew to trust her. At one point, he developed feelings for her, which stirred his heart to beat again, his powers temporarily weakened.