It's not so much that Roderick Usher exhibits female characteristics, as the question suggests, but rather that he is an eccentric, isolated soul who may or may not have engaged in an incestuous relationship with his sister, Madeline. In Edgar Allan Poe's short story The Fall of the House of Usher, Roderick has summoned his only friend, one whom he has not seen since childhood, presumably because he knows of no other individual in whom he can confide. Roderick and Madeline have lived in seclusion for years, as the former tries desperately to contend with the family history of madness that he fears has at last begun to claim these two sole surviving heirs to that name. To the extent one can ascribe feminine characteristics to him, then, it is a product of Roderick's mentally unstable status and the ramifications of having lived much of his life in such isolation with only his sister and the family valet -- with occasional visits from a physician -- as companions.
As noted, it is far from certain that Roderick actually exhibits feminine characteristics as opposed to simply being an eccentric and emotionally unstable individual. Clearly, Poe intended Roderick to appear more than a little unusual, as when the narrator is first greeted by his host upon arrival at the Usher mansion:
"Upon my entrance, Usher arose from a sofa on which he had been lying at full length, and greeted me with a vivacious warmth which had much in it, I at first thought, of an overdone cordiality—of the constrained effort of the ennuyé man of the world."
One could interpret this description as suggesting a certain femininity on the part of Roderick. Certainly, to contemporary readers, that would appear to be the case. Poe, and his characters, though, lived in a different time. Roderick is refined and educated, a man whose passions include literature and music, the latter no longer available to him by virtue of his affliction. Roderick's social and cultural refinement, however, does not by itself deflect attention away from Poe's physical description of the man who has invited the narrator to hear of his woeful state. The narrator provides a detailed physical description of his childhood friend that does indeed suggest a certain femininity:
"A cadaverousness of complexion; an eye large, liquid, and luminous beyond comparison; lips somewhat thin and very pallid but of a surpassingly beautiful curve; a nose of a delicate Hebrew model, but with a breadth of nostril unusual in similar formations; a finely moulded chin, speaking, in its want of prominence, of a want of moral energy; hair of a more than web-like softness and tenuity . . ."
Not only does Poe provide an atmosphere and story that could involve an incestuous relationship between brother and sister, but he also provides details that could suggest a homosexual relationship between Roderick and the narrator. Very early in the story, the narrator references what had once been an "intimate" relationship, albeit one between young boys, and this latter physical description provided above only reaffirms the notion that Roderick may be a homosexual. It is the narrator's description, though, that makes such a suggestion credible. It is the narrator who uses the phrase "liquid and luminous" in describing his friend's eyes and "surpassingly beautiful" in describing Roderick's lips, and so on.
Roderick Usher could be homosexual, bisexual, heterosexual, heterosexual but having engaged in an incestuous relationship with his sister, or simply an emotionally troubled individual with no particular sexual orientation or proclivities at all. Those physical descriptions, though, provide the basis for a thesis centered on Roderick's eccentricities and physical features.