the greatest of defences, they are merely symbolic of the other side to her nature.

The flower tells him to go, for she does not want the prince to see her cry as she is proud.  Without the prince, however, she will be vulnerable.

The sheep is a complex symbol in the story. The little prince is not satisfied with the pictures the pilot draws of the sheep until the pilot makes a picture of a box with airholes. The prince is then satisfied, symbolizing that overtelling or over-describing interferes with the imagination and the ability of observers to fill in the experience they need. However, the sheep also symbolizes concrete material need—the need that convinces the pilot the little prince is real:

If anybody wants a sheep, that is a proof that he exists.

The little prince needs the sheep to eat the Baobabs that threaten his rose, so the sheep also becomes a symbol of the prince's desire to protect the one he loves. The sheep becomes the equivalent of wearing a sword.

The rose symbolizes love. The little prince learns that even though there are millions of roses that look just like his, his is of supreme value because of the relationship he has with it.

Adults symbolize lack of imagination and vision. Their childlike imaginations are ruined as they become fixated on the parts of life that are, ultimately, less important, such as making money, being in charge, or owning things.

The snake represents the devil. As in the biblical story, the work of the devil kills Christ, but this seemingly evil deed allows for his resurrection. The little prince hopes for a similar rebirth, in his case on his tiny home planet, when he allows the snake to bite him. In this context, the snake/devil is not evil but just following the dictates of his nature.

It can be hard to attach specific meanings to the things you mention. Let's consider each one:

The sheep is not even a character, really, but a desire, or, at best, a drawing. It is important that the only satisfactory drawing of the sheep is not a drawing of a sheep at all, only of a crate containing the sheep. So we can understand the sheep as a symbol of unconscious desire, or of the imagination -- it something we want but can't really describe in a direct way. The sheep can also be understood as a religious symbol: for Christians, Christ is known as "the lamb of God." The picture of the crate with the sheep inside can be understood as representing faith -- like the sheep, faith is a belief in something you feel to be true, but can't see.

The rose represents love. Flowers often represent beauty, grace, and purity, but the Prince's rose also is vain and demanding. The Prince's problem in the book is his quest to understand the Rose, and to understand how to love it. 

The snake, like almost everything in the book, can have many meanings. In a Christian sense, the snake has a specific meaning as a form of the devil that tempts Eve and causes Adam and Eve to be cast out of Eden. The snake represents death. Yet in the book, the snake is the means for the Prince to return to his planet -- a means for a sort of ressurection. In this sense, it is another kind of savior.

The adults represent the everyday world in which no one has time for imagination and everyone is consumed with useless tasks. There is the geographer that can record no information because he is not an explorer, or the drunkard who drinks because he is sad and sad because he drinks. All the adults are stuck in similar ruts, and none of the things they care about have any bearing on the real issues in the book.

So you can see that it is not easy to attach specific symbolic meanings to elements in the story. I think part of the point of the book is to make it difficult to create those kinds of definitions, as a way of challenging the reader to engage his own imagination!

The symbolism of the sheep can best be understood if we first recall some of the lyrics found in the oratoria Messiah composed by the great George Frideric Handel:

All we like sheep have gone astray-ay-ay-ay-ay-ay-ay-ay-ay-ay. We have turn-ed, we have turn-ed everyone to his own way; everyone to his own way.

Translation: "All we like sheep have gone astray. We have turned everyone to his own way."

While Saint-Exupery may not exactly have had Handel in mind when he wrote both the opening chapters and the ending to the Little Prince, he certainly did have biblical allegory in mind. Biblical allusions can be seen all throughout the book. The snake that kills the prince is the same biblical serpent that symbolizes death in the Garden of Eden. It has also been noted that the pilot is lost in the desert just as the Israelites were lost in the desert coming out of Egypt. In addition, in the 25th chapter, both the pilot and the prince go in search of water and find a well unlike the wells in the Sahara that are "mere holes dug in the sand" but instead "like a well in a village," which is symbolic of Christ's well in Samaria and living water (eNotes, "Literary Precedents"). Finally, the prince experiences a transcendence. Not only does he die to reach a higher plane but also experiences a sort of resurrection as his body is not found at sunrise the next day, symbolic of Christ's transcendence and resurrection.

Hence, the sheep the prince asks the pilot to draw in the very beginning of the story is also a biblical allusion. We must remember that at the moment the pilot meets the prince, the prince has returned to the desert in order to make his way back home, a home that the prince ran away from a year ago. Feeling lost, confused, and rejected by his flower, the prince left his home to try and gain new understanding. He went astray. He ran from the one thing that was important to him, love. And now like a sheep returning to the fold, he realizes that he has acted as a sheep and gone astray; therefore, he is asking the pilot for a drawing of a sheep to forever remind him of the journey he has had as a lost sheep and to remind him of his spiritual transformation.

It becomes evident that the sheep is not only symbolizing spiritual floundering but also spiritual transcendence when the prince dies. Like the Lamb of God, the prince must experience death to be returned to his point of origin. The symbolism of the prince as the Lamb of God is also evident earlier when the prince first meets the snake who calls the prince "innocent and true" (Ch. 17).

Hence we see that the snake symbolizes both spiritual floundering and spiritual transcendence and that the prince is meant to represent the Lamb of God.

In the beginning, the sheep is simply another source of curiosity for the little prince. He was aware of the existence of sheep and wanted to learn more about them. In his openness to knowledge and his willingness to see and understand with his imagination, the "very small sheep" in the box is a perfect outlet for his creative love.

The pilot comes to understand that the sheep, much as the little prince loves him, represents a threat to another beloved possession of the prince's - his rose. The prince is horrified to realize that the sheep might eat the rose, just as it eats grass and small bushes.

If some one loves a flower...He can say to himself, 'Somewhere, my flower is there...' But if the sheep eats the flower, in one moment all his stars will be darkened...

The sheep represents the dual nature of true love and of great commitment. In giving ones devotion to anything, one receives great pleasure and reward but also takes the risk of being deeply saddened or hurt.