Complex sentence: The only way to return him to his proper form is with a kiss from his most beloved maiden, whom he quickly determines to be Airi and is always quick to proclaim is everlasting love for her, much to Airi's chagrin at first.
Split into simpler sentences: The only way to return him to his proper form is with a kiss from his most beloved maiden, whom he quickly determines to be Airi. He is always quick to proclaim is everlasting love for her, much to Airi's chagrin at first.