Layla Hassan is the Animus user who frames the modern-day story in Assassin's Creed Valhalla.
Layla uses the Animus to experience the memories of Eivor, a Viking who leaves Norway and becomes involved in the political struggles of ninth-century England. The modern-day sections connect Eivor's memories to the continuing conflict between the Assassins and Templars and to the series' larger mystery about precursor technology.
Layla first became a major playable modern-day character in Assassin's Creed Origins and also appears in Odyssey. Valhalla continues her storyline rather than introducing an unrelated modern protagonist. The Animus itself is a fictional machine that reconstructs genetic memories; it does not literally send a user into the past.
Desmond Miles is the central modern-day character in the first major arc of the series, while Rebecca Crane and William Miles support the Assassin cause. Those characters are therefore plausible alternatives, but Valhalla's principal modern-day user is Layla Hassan.