C.S. Lewis was the writer who co-founded the Inklings with J.R.R. Tolkien.
The Inklings were an informal Oxford literary discussion group active mainly during the 1930s and 1940s. Tolkien and C.S. Lewis were central members, and the group met at Oxford locations including The Eagle and Child pub, known to members as the Bird and Baby, and in Lewis’s college rooms.
Members read works in progress aloud and discussed literature, mythology, religion and narrative craft. Tolkien shared parts of The Lord of the Rings, while Lewis developed works that would become important in his own fiction and Christian writing. The group was not a formal society with a fixed membership list or constitution.
Charles Williams was also an important Inkling, which can make this question confusing. He was a member, but C.S. Lewis is the key co-founder and Tolkien’s close intellectual friend associated with the group’s formation.