When the Buddha died, the community had no appointed successor and had been told to take the teaching and the discipline as its teacher. Within days there was a problem: an old monk was heard saying that they were now well rid of the great ascetic and could do as they liked. Mahakasyapa heard it, said nothing at the time, and then acted.
He convened an assembly at Rajagriha, five hundred senior monks, in the rainy season after the death. Ananda, who had heard everything as the Buddha's attendant, recited the discourses; Upali, the former barber, recited the rules of discipline; and the assembly settled what the tradition would consist of. Ananda was famously admitted only at the last moment, having attained the required realisation the night before, and was then made to answer for a list of faults including having asked for women to be admitted to the order.
Mahakasyapa is remembered as foremost in ascetic practice, keeping the optional austere observances, living in the forest, wearing rag robes and refusing invitations, and continuing all of it when he was too old for it. The canon records the Buddha telling him gently that he might now take an easier life, and Mahakasyapa declining, on the ground that later generations would need the example. That exchange is the character in miniature: rigid, and right about the reason.