In Kingdom Hearts, Riku was supposed to be the Keyblade master. Since his heart fell to darkness, he did not have the chance to wield the keyblade except for a brief segment of the game. Instead he became accustomed to the nameless artificial Keyblade that was created using the hearts of the seven Princesses. It has the power to unlock a person's heart. It's black & red, and the edge forms the shape of a heart. Needless to say, it symbolizes darkness. This keyblade is destroyed when Sora uses it on himself to free the hearts of the seven Princesses.
Henceforth, Riku's main weapon of choice is the Soul Eater, a sword whose blade is curiously shaped like a dark wing. There is also what appears to be a blue eyeball near the hilt. The wing's colors are red and blue, which I believe to be symbolic. Blue may represent the sky & the sea, meaning Sora & Kairi if you pick out the references in their names. Red may symbolize love, as in the love for his friends. Therefore the wing is what Riku desires -- to be with his friends. Yet it is but a wing, and Riku knows not how to fly. He may have been an angel at one point, but all that remains is a single dark wing. He simply keeps his friends in mind to continue fighting. Symbolically they are by his side all the time, even if he is all alone now. As for the blue eye, it seems to be sort of evil eye. Perhaps it represents that Riku is always being watched by darkness, or rather, Ansem himself.
Way to the Dawn is the name of Riku's keyblade, which he acquires later on in Kingdom Hearts II, after he has banished his dark powers for good. The name may sound familiar since his last words in Kingdom Hearts: Chain of Memories were "The road to dawn." Now this makes his weapon's name significant. It refers to his path, which lies inbetween the darkness and the light. Also, since the term "dawn" is used, we see a reference to the sky Sora. Sora, in turn, represents light, as that is the element of his powers. Therefore Riku's keyblade is the way to the light. This makes sense because at the time that he gets his keyblade, he has abandoned all of his dark powers, and is left with only light. As for its appearance, it looks exactly like Soul Eater except for a few additions. The Heartless emblem makes up the key chain at the end, symbolizing his connection to Ansem, the seeker of darkness. The dark wing and light wing each make up half of the side handles. Lastly, a white wing is added towards the tip of the blade. Now again with the symbolism. The new light wing now balances out the dark wing, giving Riku a complete set. Physically the keyblade portrays light and darkness, both of which Riku was able to wield.