Helena Bahbem is Ernst von Bahbem's niece. As a child, Helena often took advantage of her position and used to get away with anything she wanted. Indeed, she was a spoiled little brat and much of her obnoxious pride sticked with her as she grew into an adult. Helena was raised as a potential instrumentalist, but apparently she got too old before she fully awoke as a Mulian and no longer could fulfill her promise as an Ollin. Consequently, she feels jealousy for anybody who succeeds as an instrumentalist, including Ayato and most especially Quon, since she is the destined Ollin of the black RahXephon, the job Helena was most likely after.
Helena seems fully aware of her surroundings, and can even see when the RahXephon has manipulated time. Despite all this however, she knows that she's just another of Bahbem's tools and often feels exasperated whenever she's around Bahbem. At the end of the series, Bahbem switches bodies with her, leaving Helena in the body of the old man. At this point in the story she's completely humbled, but gets shot in the head by Makoto who think it's still the regular Bahbem.
You can read an interesting tidbit about Helena in Question #7 of the FAQ.