He is often credited with creating X-23, along with Craig Kyle, for X-Men: Evolution. However, Yost gives the credit to Kyle for creating her while he just wrote the episode "X23" she initially appeared in. The character did prove popular enough to be adapted into the Marvel Comics and other media.
Yost began working for Marvel in 2002 as an intern. His scripts caught the attention of Marvel executives who hired him to write for X-Men: Evolution. He began writing for Marvel's X-Men comic book and even wrote for DC Comics.