It's officially announced. The Last Seiken's Rider is KAMEN RIDER DURANDAL. Kamen Rider Durandal is Reika Shindai's Elder Brother - Ryoga Shindai. He & Reika work under the direct orders of Master Logos. Durandal & Sabela are promoted as the First Brother & Sister Rider in the History.
As per Wikipedia,
Durendal, also spelled Durandal, is the sword of Roland, as legendary paladin and partially historical officer of Charlemagne in French epic literature. It is also said to have belonged to young Charlemagne at one point, and, passing through Saracen hands, came to be owned by Roland.
Ryoga's Seiken is actually a Trident. But in actual life Durandal (sword) doesn't even look anything like Trident.
Ryoga Shindai is played by Ken Shonozaki who is celebrating his 10th anniversary of his acting career.