Spoiler warning: 7th volume
When Mika looks at things, she sees possibilities. When she looks at people, she sees even more of these. Her mind is in a constant planning mode, reflecting upon where to go, how to look, who to keep tabs on, what place to bug, what to hack, what mysteries to pry out of people's minds and piece together from jumbled words.
It seems she's constantly at Seiji's side. But it only seems so because when she leaves it, he stills and sits in place, staring at the wall. He's gullible like that after years of Namie's control, too tight to let him develop any will of his own other than the feverish need to fight for his bodiless love's sake.
Mika loves Seiji. But it isn't the stupid selfish kind of love that would make her want to have his affection for herself forever if he doesn't really want that. She wants to make his dream come true.
She's investigating, investigating, investigating. Because that's what she's best at. Because she's a stalker at heart, she's interested in everyone's matters and she will stop at nothing to get a clear image of everything that's happening in this city.
Seiji's silent affection is just a break from a constant operation she undertakes like some kind of a special agent, keeping tabs on everyone and everything, every single day.
She rarely speaks to Mikado, even though he is in her class, but she's hacked into his computer and gets notifications onto her phone every time he posts on the Dollars forums. She snatches the logs of his chats every day at 2 pm and reads through them before sleep.
She has a shark mask she carries around in her purse and puts it on to mingle with the Blue Squares moving around on the streets. She bugs Kuronuma Aoba's apartment on his second day of high school and regrets she hasn't done that earlier.
She replaces the bugs in Anri's apartment regularly because they run out of batteries approximately once a year.
She buys herself a yellow scarf when Kida Masaomi comes back to Tokyo and reinstates his gang.
Every single time she puts bugs at Orihara Izaya's office, he removes them. But she's seen the valkyrie's head there with her own eyes.
When the final battle starts on the streets, she follows Celty-san to his place. She gets in just in time to witness her head reconnecting with her body in a column of blinding light.
She's heard Shinra-san talking about how anaesthetics don't work on Celty-san through the bugs at his place. She also knows Celty-san regenerates from injuries after a temporary weakening. In such a case, she sees nothing wrong with shooting her right in her heart with a sniper rifle.
She got it from that Russian girl's Vorona's place she's been listening on for some time now, learning herself Russian in the process.
"This person belongs to Seiji-kun." She informs Izaya who puts his hands up when she points her weapon at his head. "I will give her to him. Because I love him that much."
She drops the rifle and his switchblade is out and open the very moment she does. She shoots him with a tranquilizer gun before he can get close though. Unlike some people, he's not special in a way that would make it not work on him and his body slumps down to the floor.
She brings the valkyrie to Seiji and witnesses the two of them disappearing together in another column of blinding light. Right before that happens, his smile while he's in her embrace is unforgettable.
Mika's happy with Seiji's happiness.
Now that Seiji's no longer there and can't get into trouble, she makes a few phone calls that clear up a lot of things for a lot of people and the war on the streets stops within a few hours.
She goes to school the next day as if nothing happened.
There are rumors about Mikado having dropped out.
She sits down at his abandoned place next to Anri and smiles at her.
"Nee, Anri-chan, now that Seiji-kun's gone I think I need to find a new love interest." She speaks up in an innocent voice of a high school girl.
END
I'm afraid this is just how Durarara will end, for real :|