Rich Man is an important story quest NPC on Bobby Island. He is the son of a Sick Man. He gives a quest to beat Mob Leader and return his riches back. Completing this quest will give you a relic, which can be used to unlock access to Saber Expert. Defeating him will gives you the legendary sword Saber. You need to give sick man water in order to talk to him. Please note that upon talking to him without doing the sick man quest first would just result in him asking you to leave.