Rishabhadeva (Rishabhanatha) attained omniscience (kevala jnana) after intense austerities. According to Jain tradition, he spent a very long period in deep penance, and after destroying his karmic obstacles (ghati-karma), he realized the true nature of the soul and became a kevalin. This supreme knowledge enabled him to teach right faith, right knowledge, and right conduct to others and to establish the Jain community.

Note: Jain traditions describe these events with some variation between Digambar and Śvetāmbara sources, but the core idea remains that he achieved kevala jnana through severe tapas and then guided others. You can read more here