In Jonathan Livingston Seagull, Sullivan is Jonathan's instructor in seagull heaven. At the end of Part One, two special seagulls take Jonathan into the next seagull world, where he will continue to hone his flying skills under the tutelage of an instructor.
At the beginning of Part Two, Jonathan finds that his seagull body has miraculously transformed into a lighter, brighter version of his old one. Although he is ecstatic at this new discovery, he is disappointed to realize that there are limits to flight speed even in seagull heaven. He is also a little discouraged when he observes that seagulls still get tired in their new home. However, things start to look up for him when he notices that he is in the company of seagulls who are focused on reaching perfection in flight.
One day, Jonathan asks his instructor why there are not more seagulls in seagull heaven. Sullivan answers that Jonathan is a 'one-in-a-million bird,' and that most seagulls often fail to grasp that there is more to life than 'eating, or fighting, or power in the Flock...' Sullivan tries to explain that most seagulls don't have the right mindset to be admitted to seagull heaven; they have to go through many lifetimes before they realize the true purpose of a seagull's life. Jonathan, on the other hand, has understood that perfection in flight is the ultimate goal.
Eventually, when Jonathan manages to master flying to the past and to the future, he finds his body changing once again to accommodate his new ability to recognize the meaning of kindness and love. To his great awe, his new body glows so brilliantly that it is a blinding vision of luminosity. This new perfection so inspires Jonathan that he bids farewell to Sullivan and proceeds to make his way back to seagull earth. His purpose is to seek out similar Outcasts and to teach these Outcasts that they too, can transcend time and space to arrive at perfection in flight and love.