You have to look at the symbolism of it. An angel is a being that is high above you, it is a messenger from God and all you can do is take in its message with a believing heart. But a devil is something that is constantly at your side always taunting, always teasing, always tempting.
Reis' place was at Gendos' side as part of his grand design Shinji could do nothing to sway her from that commitment, but always looked to her for help in dealing with his father, the eva and NERV.
Asuka never even spoke to Gendo. She was cast to live with Shinji, to always taunt and tease him, to always distract him from the grand design that he was supposed to serve. He riled his emotions, vied for his attention and tempted his desires.
The two characters are functionally an angel and a demon sitting on Shinji's shoulders, leading him into his fathers' agenda or pulling his mind away from it, even as he continued to serve him.
The form informs the function.