I do believe I've figured out the Luka/Teppannov arc. It was rather difficult because many vital songs to it are not actually part of the main series, but when you piece them together, it makes a whole lot more sense.
Songs: Nemuritai no ni!, Tabi ni Deyou!, Kokoro no Koe wo, Kokoro ni Koe wo O, Mukuchi na Hito, Russian Last Emperor, Oh So Russia!
From this point on it's a big mush of headcanon and theory, because the two have become so linked in my mind that it's impossible to separate them. If you want me to clarify/specify/give a reason for something, just let me know.
Luka grew up in an orphanage and befriended the young Urusei Teppannov. They bonded over the fact that both were of Japanese descent, and would listen to the radio together. Eventually their paths headed in separate directions when Luka became a child soldier. Her past goal was much like Irina's: do all that you can for Putin's sake. As she grew older, she found that she had a desirable figure, and used this to her advantage to climb the ranks. This is about the time that Irina becomes a soldier, and Luka can very much relate to her. She feels a motherly kind of attachment to the girl, and begins to doubt her loyalty to Putin. When Irina dies, it shatters, and she decides that she wants the whole world to know about the tragedy of Russia.
Luka continues to increase in rank and seduces Putin for information. She doesn't actually like him. In fact, she's completely disgusted with him, and would much rather push him away, but masks that and continues to charm him so that she can get as much information as possible. This continues for years. It sort of corrupts her and gives her a bitter kind of outlook. Because she feels that she needs to love Putin for Irina's sake, she almost does, but it's broken and forced. Eventually she gets it all prepared, and uploads it to the web. She has doubts about this, wanting simply to go back to Putin's arms and her warm bed and not risk her life like that, but dismisses them as selfish and goes ahead anyway. After taking a short nap, she dons her disguise and flees, fearing an assassination attempt.
Here is where we see Teppannov again. Over the years, he has become a very skilled assassin, commonly called a "demon" for his work. He's taken up playing guitar and hasn't really improved at Russian much at all (possible reason for his title of "man of few words"? another reason is that he doesn't really have a voice, w). He's been sent to assassinate Luka, and has a secret photo of her to identify her. Which he does on the beach she fled to, and there's that well known chase scene. However, another one of the songs hints that there's a fight around here, which caused him to be impressed with her fairly equal skills and fall in love with her. Whatever the case, when they stopped, he pulled out a gun... And out comes a rose. He asks her to run away with him, but another song hints that this is actually more of a proposal to elope than anything. Yes, Teppannov very much loves Luka, and the series makes this very plain. Particularly Mukuchi na Hito, which has this whole proposal scene that's really sweet. "Once I'm kneeling, even your ring finger. On the beginning of a special day." ~
This is about where they board the train together. There's a very distinct contrast here. Teppannov is thinking somewhat nostalgically, but looks forward to the future and trusts in fate. Luka is also feeling nostalgic, but in a bitter kind of way, and feels that everything is hopeless. She takes some pills (to calm herself?) but overdoses and ends up committing suicide. ...As Teppannov cries. Reading it here doesn't sound like much, but watching that song when you understand their story is absolutely heart-breaking. Since Luka was the transmitter and she is no longer alive, Teppannov takes on the duty with her laptop, keeping up with the main series (Rin/Len/Miku/Kamui arc).
Some other assorted notes:
- As Luka is to Rin, Teppannov is to Miku. Both have a definite connection. Luka because of Irina, and Teppannov because Miku is basically his voice. Think of it this way: Teppannov writes music and has Miku sing for him, as a Vocaloid. Mukuchi na Hito was actually "written" by Teppannov himself. You could say Luka and Teppannov have sort of producer/fourth-wall-y roles in the series.
- It's difficult to identify all the pronouns in Kokoro ni Koe wo O, but I figure Luka and Miku are involved somehow, as they're very important people to him.
- I still wonder if Putin was aware she was using him? If he wasn't, he was simply a slave to his own lust, but if he was... Perhaps it wasn't her manipulating him, but then manipulating each other? It's interesting to think about, and I'd have to look it over more before I say much.
- If Luka and Teppannov are the "producers", are they connected to the TeihenP/PutinP role? Teppannov writes music and uses Vocaloid, whereas Luka gathers information (writes lyrics?) and "transmits" it throughout the web.
- It's never actually confirmed whether Luka shares his feelings or not; she obviously has some lingering feelings for Putin, whether those are romantic or some kind of emotional trauma. However, she doesn't reject them, which says something at least.
Any questions/comments/concerns? I've been meaning to write this all out for a while now, so apologies if it comes across as disjointed or rushed.