It's the latest novel from the "Full Metal Panic!" series by Shoji Gatoh. If you're not an FMP! novel reader, you will probably be confused about the story so far.
But since you're really stubborn and you still want to know the spoiler, how about catching up to the story first:
The story for the 2nd season "Full Metal Panic! The Second Raid" came from the upper and lower volumes (volume 8 & 9 respectively) of the novel "Owaru Day By Day" (End of Day by Day).
After that, the story continues on to 4 succeeding novels:
1. "Odoru Very Merry Christmas" (A Dancing Very Merry Christmas) - Kaname and classmates, with the exception of Sousuke, go to a Christmas vacation on board a luxury cruise liner but gets involved in a seajack. Of course, Mithril goes to the rescue with Sousuke, Kurz & Mao. A new enemy organization is introduced -- Amalgam. First appearance of Tessa's estranged twin brother Leonard who's a member of Amalgam and also a whispered. Most important of all: Sousuke realizes his feelings towards Kaname.
2. "Tsuduku On My Own" (Continuing On My Own) - Kaname gets kidnapped by Leonard. He also steals her first kiss >< arrghh!!! Mithril's base is destroyed by Amalgam. Kaname and Sousuke admit their feelings to each other ^^. ARX-7 Arbalest is destroyed by Leonard's AS. Sousuke confess to the whole class about him being a mercernary soldier and promises to bring Kaname back.
3. "Moeru One Man Force" (Burning One Man Force) - Sousuke, now on his own after losing contact with Mithril, is living somewhere in Southeast Asia and participating in underground AS fights in hopes of getting a lead about Kaname's whereabouts. No Kaname-Sousuke reunion here. Sousuke gets shot and is near death when he is saved by a French reporter named Lemon.
4. "Tsudou Make My Day" (Together Make My Day) - Sousuke comes close to saving Kaname. They have their first contact through a radio transmission. Kalinin is now associated with Amalgam. Here comes the new mecha ARX-8 Laevatein.
So here's where my spoiler comes in:
Tsudou Make My Day (TMMD)
Chapter 11 (Final chapter)
Radio transmission dialogue between Sousuke and Kaname
Kaname: I'm ordering you as the student council vice president, okay? Come and get me. I don't care what kind of sacrifice it takes. I don't care how many people die - hundreds, tens of thousands, hundreds of millions. Just come and get me! Use all your completely useless, anti-common sense, bothersome soldier skills, and no matter what kind of dangerous opponent you face, beat them up so you can come and embrace me! You can do it. Right?
Sousuke: That's right. I can. I'm going to find you. Wait for me.
Kaname: I will... Sousuke... I love you.
Sousuke: I love you, too.
Kaname: I'm happy... so, when we see each other again, let's kiss. We absolutely have to. No matter where we are. Okay? Promise me?
Sousuke: Yeah, I promise.
Kaname: I'll wait years, even hundreds of years, so...
Sousuke: Don't worry. I'm going to find you, no matter what.
From
Animesuki FMP! thread What can I say? I've been waiting for *that* for so long now. I can't wait for the kiss too. The next novel will finally end the main story although according to Gatoh's blog, he will split the ending into shorter stories.
Translations of the novels from "Owaru Day By Day" to "Tsuduku On My Own" can be found on
Boku-Tachi or
here.