Who persuaded Buddha to allow women into the sangha?

The correct answer is Ananda.

According to Buddhist tradition, Mahapajapati Gotami (the Buddha's foster mother and aunt) wished to join the monastic order (sangha) and sought ordination. Initially, the Buddha refused her requests multiple times. However, the Buddha's close disciple and personal attendant, Ananda, intervened on her behalf.

Ananda pleaded with the Buddha, asking whether women were capable of attaining spiritual enlightenment (arahantship) just like men. When the Buddha confirmed that they were, Ananda argued that women should therefore be allowed to join the order. Moved by Ananda's advocacy and reasoning, the Buddha consented to allow women into the sangha, establishing the order of bhikkhunis (nuns), subject to certain additional rules (the Garudhammas).