Who was Achilles in Tennyson's "Ulysses"?

Tennyson refers to Achilles from Homer's Odyssey as a sort of motivational myth for the old seafaring men. He addresses the mariners and motivates them to seek unexplored avenues. Time is not in their favor as they have grown old. Yet they may be capable of something noble and noteworthy. Though they are weak in bodily strength, they were strong in cerebral and intellectual ability. In addition, they were blessed with iron will. Here, Tennyson differs with regard to Homer’s Odyssey, as the followers were dead and Ulysses returned to Ithaca alone. He attempts at mythifying their journey with references to “Happy Isles” that signifies the Paradisal paradigm where the great Greek warrior Achilles reached after his death.