Complex sentence: The only major completely fictional baseball character is the left-handed pitcher Allen, a teammate who Jack has little use for, but who eventually introduces Jack to Florence ("Florrie"), who is Allen's sister-in-law.
Split into simpler sentences: The only major completely fictional baseball character is the left-handed pitcher Allen. Allen is a teammate who Jack has little use for, but he eventually introduces Jack to Florence ("Florrie"), who is Allen's sister-in-law.