Who is Hikaru, really?
In the final episode Hikaru sacrifices herself to save Tsubasa (and incidentally the rest of humanity). Tsubasa survives, but she must go on without the person on whom she has come to depend most. This seems like a rather dark ending for a series that was never presented as a tragedy.
I am going to argue that the ending is actually a good deal more positive than it seems at first glance.
A key hint comes at the end of the episode, when Tsubasa’s father tells her that he and her mother wavered for a long time on whether to name her “Tsubasa” or “Hikaru”. To say that “Hikaru” is an alternate name for “Tsubasa” is to say symbolically that Hikaru and Tsubasa are the same person.
Symbolism is all very well, but what could that mean literally? Consider that Hikaru is actually an alien weapon system. A Ribers is designed to communicate telepathically with its operator, but it is not supposed to be true individual with a unique personality. (That is why Orudina tends to treat Hikaru as an object with no rights.)
There is only one way that Hikaru could have acquired a human personality, and that is from Tsubasa. Hikaru is actually a buried part of Tsubasa’s personality which Tsubasa has never been able to express: the bold, cheerful, confident traits that she inherited from her mother.
Remember the way that Hikaru first introduces herself to Tsubasa: “I am the other you.”
Before she dies, Hikaru helps Tsubasa get in touch with this unknown part of herself.
This raises some deep questions about the nature of identity which I am not able to answer. However I think the point of the story is that Hikaru lives on in Tsubasa in a very literal sense, and that her wish “to be with Tsubasa forever” was in fact granted.
What is the symbolism of the OP and ED?
Usually white clothing means that the character (other than a bride) is dead or in spirit form. Based on that, it probably means something like “together, forever, in spirit.”
This is probably meant to be taken figuratively. However the way they are shown frolicking in the Hokkaido countryside in all four seasons suggests another more literal interpretation: because Hikaru died defending the land from an existential threat, the two of them became guardian kami for the region. This may seem like a bit of a stretch, but it is common practice in Shinto to enshrine national heroes.