Start by watching, not optimizing
The visible rules are rock, paper and scissors. The more important clues are the opponent's tone, pauses and reactions. Treat the first run as observation rather than a perfect-score attempt.
Save your conclusions for the second run
The official project description says the current route contains several endings and that Nate may not play fairly. One loss is information. Avoid assuming a hand that worked once is a permanent solution.
Keep the official build current
Download the demo from the creator's itch.io page. The project is in development, so dialogue, compatibility and route behavior can change between builds.