In the series, Simmons is one of S. H. I. E. L. D. 's top scientific minds. Though her experience is vast, she is particularly expert in biological sciences. Many of her storylines involve her relationship with her best friend, and later lover, Leo Fitz. Over the course of the series, she develops from a relatively young and inexperienced S. H. I. E. L. D. scientist to one of S. H. I. E. L. D. 's most senior agents, also clocking significant experience as a field agent. She is distinguished from her colleagues by taking very determined and sometimes coldly rational decisions in the pursuit of what she believes is right.