Stephanie McBain is the daughter of President Lee McBain, who was captured by invaders and remade into the alien entity Xaerous Brain.

Kidnapped during a peace treaty between Gradius and Bacterian factions, Stephanie was forcibly converted into a cybernetic host brain for the alien leader Gofer, destined to command the Bacterian forces against her home planet.

Harboring romantic feelings for fellow pilot Dan, these emotions influence her choices during critical moments. Following her capture, Dan and Lord British spearhead a daring rescue mission to extract her from the depths of the Bacterian Artificial Sun facility.

Liberated, Stephanie joins the battle against the invaders. She confronts and destroys her father, now the Xaerous Brain, defending her planet alongside Dan and Eddie. This decisive act directly thwarts the Bacterian invasion plans.

Her ordeal as a captive and her subsequent actions in the conflict shape her path, culminating in her pivotal role during the final confrontation against the alien forces.