Guillermina Massanet Rivera de Fermaintt was, without any doubt, the most important figure in Spiritism in Puerto Rico during the last century. She distinguished herself through her dedication to promoting a form of Spiritism that remained more faithful to the works of Allan Kardec. Born in 1892 in the town of Arroyo, Guillermina moved to the capital, San Juan, while still young in order to complete her education. She graduated as a teacher from the University of Puerto Rico and devoted many years to the teaching profession. Within Puerto Rican society, she was also well known for her involvement in social and cultural initiatives. In her personal life, she married Benigno Fermaintt, and together they had three children.

Her outstanding contribution to the Puerto Rican Spiritist movement led to her appointment in 1946, during the presidency of Luis Sánchez, as Vice-President of the Federation of Spiritists of Puerto Rico (FEPR). That same year, Sánchez and Guillermina attended the First Pan-American Spiritist Congress, held in Buenos Aires, Argentina, where the Pan-American Spiritist Confederation (CEPA) was founded, and the FEPR officially became affiliated with the new international organisation.

Some time later, following Sánchez's resignation, Guillermina assumed the presidency of the FEPR, a position she held until 1970. In 1957, she became the first woman to be elected President of CEPA, serving until 1960. Upon taking office, she inherited an organisation divided between those who supported and those who opposed the definition of Spiritism as an essentially scientific movement without a religious character, a position that CEPA had adopted in previous years. Seeking unity and a spirit of reconciliation, her administration was marked by the drafting of a new constitution that helped reduce these differences.

Later, in 1969, Guillermina served as President of the Eighth Pan-American Spiritist Congress, held in San Juan, Puerto Rico. In 1977, accompanied by other women, she travelled throughout Puerto Rico, visiting numerous Spiritist societies to seek support and contributions for the establishment of the Puerto Rican Spiritist Women's Association in Action (AMEPA). Her efforts were successful, and on 17 March 1977 AMEPA was founded, with Guillermina elected as its first President.

She returned to the spiritual homeland in 1984 at the age of 92. In recognition of her contribution to the development of secular and free-thinking Spiritism, Guillermina has been honoured on several important occasions in CEPA's history. In 2008, for example, the Organising Committee of the Twentieth Pan-American Spiritist Congress, held in Puerto Rico, dedicated the event to her memory. Her greatest contribution to the Puerto Rican and Latin American Spiritist movement was her steadfast advocacy for the dissemination of Spiritism in its authentic form, grounded in Kardec's codification and free from syncretism and mystification, which she regarded as essential to its continued survival.