The two-faced Roman god of beginnings and endings was Janus.
Janus governed beginnings, transitions, doorways, gates, passages, and endings. Roman artists typically showed him with two faces looking in opposite directions, a visual symbol of looking backward and forward. His name is also the source of Ianuarius, the Roman month that became January.
Janus was invoked at the start of undertakings, journeys, harvests, planting seasons, marriages, and other major changes. Roman religious thinking treated beginnings as especially important because the first words, actions, or omens could influence what followed.
A common mix-up is confusing Janus with Terminus. Terminus protected physical boundaries and boundary stones, while Janus represented thresholds and transition more broadly. Janus was a distinctly Roman deity rather than a simple equivalent of the Greek Hermes or another Greek god.