In Scene One Yank is described as the fireman's most "highly evolved individual." However, Yank's inability to deal with Mildred reveals Yank has evolved only to specifically survive the rigors of the Ocean Liner and industrial work—not to process complex, cerebral issues. The men kid and taunt Yank, repeatedly recalling the scene of Yank turning to see the ghostly Mildred. Paddy recalls Mildred's reaction, "She [Mildred] shriveled away with her hands over her eyes to shut out the sight of him 'twas as if she'd seen a great hairy ape escaped from the Zoo!"
Yank is stung by Paddy's descriptions of how Mildred looked at him. In an odd mixture of "thought-punches," Yank vows to "brain her! I'll brain her yet, wait 'n' see!" Yank threatens to kill her by a blow he head, the word choice is revealing about his character. The word "brain" can refer to the physical organ, a very smart person or killing by smashing one's skull. Yank wants to take aim at what makes Mildred smarter and superior to him—to "brain" as in to hit and also to "brain" as to be smarter than Mildred.
Unable to physically "get even" with Mildred, Yank resorts to the adolescent tactic of "belonging"—insisting that Mildred does not "belong." Mildred is inferior to the likes of Yank because he "moves," helps run the ship engine, and she's "dead." Yank reduces Mildred to "baggage" that he physically carries. Because Mildred has no physical function, because she does not help to propel the ship, she is lesser.