Richard of St. Victor was a 12th century theologian living in Paris just before the creation of the very first university there. He, like many of his contemporaries, was deeply influenced by the writings of Augustine. Much of his time was spent participating in the liturgical life of his community: the Victorines. This was a group of canons regular founded by William of Champeaux and Hugh of St. Victor who lived in accordance with The Rule of St. Augustine. Without going into too much detail, the Victorines not only sang hymns and prayed prayers together, they also brought an intensity and ingenuity to the composition of their hymns and prayers, as well as to their theological works. Richard is most known for bringing this ingenuity to his work on the Trinity, where he sought to prove—through the contemplation of Divine love—that the Divine, though one, must also be a plurality of persons; for the highest, most integral, and fullest love requires at least, and no more than, a Trinity of persons. Richard wrote many other works as well, most of which focus on the moral and spiritual formation of his community.
The reason you’ve never heard of Richard of St. Victor is likely because of a few factors. One, he gets overshadowed by other theologians who are more well-known. St. Thomas Aquinas, for example, lived a generation after Richard, and he knew of and read some of Richard’s works. A second factor is that theologians like Aquinas either didn’t know of Richard’s work on the Trinity or did know of it but didn’t make it the basis for their own articulations of the Trinity; St. Bonaventure, by contrast, seems to have been more influenced by Richard’s trinitarian theology than Aquinas. Both Aquinas and Bonaventure are more well-known than Richard, in part because of the rise of the schools and the theological controversies that ensued there; also, each has deservedly earned a prominent place in the canon of great theologians. A third factor is a more practical one: much of Richard’s work has not been translated from the Latin. Grover Zinn’s English translation of Richard’s three main works was published with Paulist Press for the Classics of Western Spirituality series in 1979. As people become more familiar with Richard, more and more are studying his theology and explicating his written work.