Mentor stands as representative of the ancient Arisian species, evolved in one of two colliding galaxies long before the narrative events. These beings possessed profound mental capabilities, dedicating themselves to wisdom and nurturing nascent civilizations across the cosmos. Their existence placed them in direct opposition to the malevolent Eddorians, originating from the other colliding galaxy, who thrived on conquest and subjugation. This fundamental conflict between Arisian benevolence and Eddorian aggression defined Mentor's purpose.

To counter the Eddorian threat, Mentor initiated a meticulously orchestrated breeding program spanning millennia, designed to cultivate individuals capable of wielding the Lens. This unique telepathic technology embodied the pinnacle of Arisian mental science, functioning as a symbiotic device attuned to its bearer's life force, granting advanced telepathic abilities while proving fatal to unauthorized users. Mentor personally oversaw the selection and training of key individuals, including the first Lensman, Virgil Samms, and later, Kimball Kinnison. The Arisians deliberately obscured their intervention from emerging civilizations, like humanity, to avoid premature revelation of the cosmic conflict.

Mentor's direct interaction occurred when Kimball Kinnison, stranded near Arisia after escaping a Boskonian attack, experienced an out-of-body encounter. During this, Mentor implanted Kinnison with the Lens, designating him pivotal against Boskone—the interstellar criminal organization serving as an Eddorian proxy. Mentor emphasized Kinnison must independently master the Lens's capabilities through experience, offering only minimal guidance about its significance. This established Mentor as a cryptic guide enabling key agents while withholding comprehensive knowledge of the larger war.

Throughout the ensuing conflict, Mentor operated indirectly, allowing Lensmen to confront Boskonian forces while intervening only to ensure critical strategic outcomes aligned with Arisian long-term objectives. For instance, Mentor permitted the defeated Boskonian leader Helmuth to retain sanity after an ill-fated journey to Arisia, explicitly stating this served Civilization's interests, not mercy. This illustrated Mentor's calculated approach. Later, during Kinnison's confrontation with the Eddorian entity Gharlane on Thrale, Mentor covertly assisted, masking Gharlane's true form to prevent premature awareness of the Eddorians. Such interventions demonstrated Mentor's commitment to a gradual revelation of the cosmic war's scope, ensuring Civilization developed the resilience for ultimate victory.

Mentor's guidance extended to approving significant personal milestones for key agents, such as authorizing Kimball Kinnison and Clarissa MacDougall's marriage after Gharlane's defeat. This endorsement underscored the Arisians' understanding of biological and emotional factors within their breeding program, leveraging human relationships to advance genetic objectives. Following the eventual triumph over Eddore, orchestrated by the culmination of the breeding program in the Kinnison children, Mentor and the Arisians departed the universe, entrusting its guardianship to the perfected Lensmen. This fulfilled Mentor's foundational purpose: creating a self-sufficient inheritor species capable of sustaining Civilization without external guardianship.