Matsuo Basho is a legendary japanese poet and Zen Master. His most famous works, like The Narrow Road to the Deep North, revolve around accounts of his travels on foot across Japan. In each of these journeys, he was accompanied by a different companion. Despite the fact that Basho spent literally every day with these young men for months at a time, little to nothing is said about them other than noting their presence and recording a few of their poems.
Who were these men?
Why did they accompany Basho?
What was the actual nature of their relationship, can we guess that from cultural context of the period?
Do we know anything about their history or what they did after these journeys?
First, a correction: Basho is not and never was known to practice Zen, much less be considered a master of it. This reputation comes from later criticism that argued his work embodied the values of Zen. He frequently commented on the fact that he was not any kind of monk in his literary works, while wishing he had the strength to give up poetry and become one. All that is known is that he had a brief friendship with a Zen roshi named Buccho, that he often quoted Daoist texts directly, and that the poet he most admired, Saigyo (12th century), was a devout practitioner of Kukai's Shingon Buddhism, which is a tantric (esoteric) school. All of this is in line the conventional literary training of the well-read in 17th-century Japan, not any particular inclination towards Zen.
As James H. Foard observes:
There is no evidence that Basho had more than a very general popular understanding of Buddhism, filtered through his reading of ancient literature and No drama. He would draw upon this understanding during visits to famous Buddhist temples, but only as part of his general concern with historical and literary background in every place he went. At one point in Nozarashi Kiko, he seems irritated when he cannot get into the Ise Shrine because he is mistaken for a Buddhist priest. Basho had briefly flirted with Zen in Edo under a master named Buccho. This priest, however, moved away in 1682, and, while Basho remained fond of him, it is impossible to think that he practiced Zen for more than the shortest period of time.
Now, for his companions, are you asking about those he meets in Narrow Road specifically, or across his life? If the former I'll need to dig out my books. For the latter, it's important to note that his large number of companions is in part due to his frequent breaks with old students when he abandoned his previous aesthetic theories, and they were unwilling or unable to follow his new methods. This also accounts for the number of schools which sprouted after his death, all claiming to correctly teach his style and theories.
If you have specific questions about any individual companions, I might be able to give you a more satisfactory answer.
Sources:
James H. Foard, "The Loneliness of Matsuo Basho"
William LaFleur, Awesome Nightfall: The Life, Times, and Poetry of Saigyo
Eleanor Kerkham, Matsuo Basho's Poetic Spaces: Exploring Haikai Intersections
Peipei Qiu, BashÅ and the Dao: The Zhuangzi and the Transformation of Haikai