The following was summarized from Microsoft Knowledge Base Article 165908.
To unlock the "Shinobi Lord" ending (True Ruler), you need three specific characters at "Trust Level 5":
At its core, Naruto- Shinobi Lord is an adult-oriented visual novel with RPG elements. Developed by the team known as Cats-creators, the game distinguishes itself from standard fighting adaptations by focusing on storytelling, character relationships, and life simulation within the Ninja World. Naruto- Shinobi Lord -v0.31- By Cats-creators