Special thanks to @thesilentknightblade for the suggestion. So! We're getting a package deal. A two-for-one special on some headcanons~
Since they asked about platonic and romantic headcanons for the duo, I'll oblige for both.
So, what's funny about SirenService is the best way to describe them is that they are best friends first, lovers second. And this follows after their first meeting.
-CS/Jonathan and Lament are childhood friends. When they first met, Jonathan had just turned 13 and Lament was going to turn 11 in that December.
-Jonathan didn't have anyone he felt comfortable or safe talking to beside his (adoptive) mother, Solah, and his (adoptive) sister, Velvela. But neither of them were close in age to him and had their own responsibilities. Solah was the high general of Nightmare's armies. Velvela was five years older than her brother and after she turned sixteen, she was fighting on the front lines of Nightmare's war. So when Lament arrived, he was just excited to try and make a new friend. All of the other demons close to his age either lacked sapience or saw him as too weak to really find any worth in interacting with him. Even his tie to Nightmare didn't work to his benefit.
-It took a while for them to get used to one another. They would talk in between their training sessions. Jonathan would show Lament some books he'd found while Lament would show him different things Solah was teaching her. They would bond over things that they were learning.
-As the years went on, Nightmare eventually intervened a little more, but it wasn't to separate them. Not intentionally. While Solah taught Lament combat skills, Nightmare helped Lament practice her siren magic. But with their trainings, it was harder and harder for them to interact as much as they wanted. But teenagers will be teenagers, and Jonathan particularly fought to try and spend time with his friend before Nightmare started to get ready to send her to the front lines.
-On the occasions when Velvela was home, the three of them would all spend time together, razing havoc in the NME Delta Base. Velvela would shift forms into a Wolfwrath (for context, she's a werewolf demonbeast known as a Lycanwrath) and the two teens would ride atop her back and run around the base and end up scaring other employees.
-Lament's role in the war effort was a spy. And she went out of her way to tell Jonathan about all the planets she'd seen and the people she visited. This tended to work to Nightmare's benefit as he made Jonathan report directly to him or sometimes Solah on some of the information.
-Jonathan's love of the arts was really spurred upon by Lament. She was the one that brought his first real sketchbook to him. Beforehand, he would draw in old notebooks, and most of those old drawings were low-effort sketches of landscape photographs and quick drafts for different machines. He had to work alongside various engineers and scientists quite regularly, on top of all his other duties.
-There is a point in the timeline where Jonathan was learning piano. I want to say that he was learning this as a young child under Nightmare's instruction and from time to time, Nightmare would also try and have Jonathan play while he trained Lament's singing voice. Oddly enough, Nightmare always had to resist her spell the stronger it got. Jonathan never had to.
[Content warning for alcoholism ahead. I will indicate when the warning is up so anyone uncomfortable can skip these points]
-As time went on and Jonathan got busier and busier, he wound up getting hooked on alcohol through Velvela. There were a few times where Lament would have to take over some of his customer service duties because he was too hungover and sick to handle them himself. He wound up in trouble at the hands of NME's head doctor, Dr. Anastasia, a few times.
-If he was sick for most of the day, Lament would go out of her way to take care of him whenever she wasn't having to handle his duties. As for why she was taking care of them, she volunteered. While they were growing up, she would visit him and watch how his training would go. While she wouldn't say she's as good a salesman as Jonathan, she has been just as successful during the occasions in which she's had to take over for him.
-Lament would go through an ongoing battle trying to help Jonathan stay away from drinking. Most customers wouldn't know he has a problem. In the present day, he's still dealing with this issue. He's simply gotten much better at hiding it as time went on. Just like with everything else he hides.
-They've both seen each other at their lowest. And what's funny, it was when they were at their lowest that they finally started seeing each other romantically.
-It wasn't until Lament nearly died from a special mission she was assigned to that they finally gained the nerve to confess. Lament had been put out of commission in the war efforts due to a stab wound that she'd received from one of her own teammates, that teammate being Yamikage. He left her to die. Velvela was the one that saved her. And it was her that urged both Jonathan and Lament to stop playing chicken and say something to the other about how they felt.
[Content warning over]
-A lot of events sort of bleed together around the time Jonathan and Lament officially get together. Lament nearly died, the war is dying down as the GSA is getting more and more overrun and overwhelmed by Nightmare's forces, and Jonathan makes a promise to Lament.
-He knew even when they were children that Lament wanted to find out about her past and who she used to be. During the war, there were times where Jonathan would have to go into NME's archives in order to catalog and organize information. During one of these times, he stumbles upon some info about a young girl named "Lucinda" that was brought in around the same time that Lament was. This girl had also matched Lament's description to a T. So, it was in the midst of making this promise that Jonathan finally told her that Lament's "true birth name" was Lucinda. But they still lacked a surname or any other information regarding her family. Even to the modern day, they're still looking.
-Dynamics didn't really change all that much between Jonathan and Lament once they formally got together romantically. They tried to keep it a secret that only Solah and Velvela knew, but unfortunately, it's one of those "worst kept secret" situations where it's painfully obvious that they're both together. Even Nightmare knows. But for some strange reason, Nightmare doesn't try and separate them...
-What Nightmare DOES do is, after the war had been over for several years and NME had full control over most of the universe, he pools some resources together for a music company. He tells Lament that he wants her to be head of NME's propaganda. Not wanting to feel the brunt of his wrath, Lament agrees and enlists Jonathan, Solah, and Velvela as members of her band. A fourth member-- an ill-mannered and sassy Jambandran named Mephistopheles Il'Soleil-- is added not too long after the first four were brought together.
-Jonathan had many masks to balance. And because of who he was, he did not feel comfortable exposing his identity in front of endless numbers of people. He invents the persona known as "Taro Karasu," in which he behaves as a mute, chivalrous pianist that communicates in sign language. The band takes on a very J-Pop/K-Pop idol inspired formula where the marketing is very parasocial and caters to very heavy, hardcore idol fans. So in that vein, in that process, Jonathan and Lament had to hide their relationship to avoid backlash from rabid fans.
-All of this ended up culminating into their friendship/relationship being quite storied and colorful. They would laugh at the most absurd points, and grumble and the most infuriating. They would banter. They would tease each other. They would go back and forth with different stories. After seeing each other at their lowest, after having been so close for so very long, there's a special level of comfort they have with one another that they don't really feel with anyone else. Jonathan loves his mother and his sister, but some things are best shared with someone outside of that familial sphere. And he has that with Lament. Lament has that with him.
So this post wound up being VERY FUCKING LONG. But there's a lot of context in their relationship that I felt necessary to share. There's a lot of blurring where their friendship bleeds into their romance. I can make a part two regarding more interactions they have with one another that go into more depth on the things they do amidst their more romantic interactions.
