Jonah ben Abraham Gerondi
Jonah ben Abraham Gerondi was born in 1200 in Gerona, Spain. He moved to Montpellier, France, where he studied under Solomon ben Abraham. When his teacher began his campaign against RaMBaM's Guide For the Perplexed, Gerondi became his most devoted assistant.
He knew members of RaBaD's family and he was related to RaMBaN.
Jonah focused on both halachah and ethics. He moved back to Spain, where he preached morality. He was particularly incensed at Spanish Jewry's propensity for keeping mistresses. He railed against the wealthy Jewish estate owners' unwillingness to help the less fortunate. He recommended that all members of the Jewish community unite to help one another, emphasizing that all Jews were responsible for one another.
He established an influential yeshivas first in Barcelona and then in Toledo. One of his most famous students was Solomon ben Abraham Adret. His repeated message for communal support for the needy and active social justice influenced the entire Jewish world and probably helped create the "shtetl" morality/mentality of later centuries.
Sources: Gates to Jewish Heritage