Complex sentence: Early in this period he met Narendranath Dutta (more familiarly called Naren), who later became Swami Vivekananda who was his close friend and who from the very beginning had made a very deep impression on his mind.
Split into simpler sentences: Early in this period he met Narendranath Dutta (more familiarly called Naren), who later became Swami Vivekananda. Naren was his close friend and from the very beginning had made a very deep impression on his mind.