Baymax is based on a character of the same name from the original Marvel Comic book series of Big Hero 6 that had inspired the 2014 Disney film of the same name.
Because Baymax's character and story is inspired by Marvel, and Disney owns a part of it, Baymax is sometimes referenced and viewed as a Marvel character. Along with the five other members of the Big Hero 6 team.
Because Baymax has a male voice, the robot is commonly viewed as one by fans.
Both Kingdom Hearts III and the Big Hero 6: The Series episode, "Portal Enemy", shows that Baymax's first body still exists within the portal.