First Look at Kamen Rider Saber Last Seiken's Rider.
He has Sharky design. He has fins on his head, near Shoulders. Whale or Shark Face on the Chest Armor. His Seiken is clearly a Trident. Most probably he is based on Mythological God Poseidon who had the ability to control seas, oceas etc.,
Last Rider is the definitely the guy who was teased in the Trailer and not Master Logos. This confirms that.