Camilla runs off in the mountains to be with her wolfskin husband + genuinely disliking royal politics (source: fire emblem birthright). The thing is, she does this and leaves behind her retainers. It was something that was make or break for her. Something either happened in court around her, or her attachment to her retainers lost to her love for this wolf.
Whatever the case, Camilla leaves. left behind is Selena and Beruka. without a lord to serve.
Selena adapts a bit better, not only due to having previous experience with being a person outside of a lord - retainer relationship... but also having her two best friends able to support her as she tries to get a position back in the higher up royal army. Being able to crash in their places or steal money from them is genuinely a good thing for her to have.
Beruka's support system was Camilla and Selena, and now both of them are gone.
From this, Beruka knows she doesnt like the streets. So she starts to hover around the knights, which some people viewed with some pity. To them, she seemed aimless. Just someone who is good as a weapon, so she was good in the army, but not much outside of that.
It is around this time, where Corrin is officially announced dead, that Silas finds himself drawn to her. Not out of romantic interest- just a genuine curiosity for the girl who looks as lost as he feels. His entire purpose for becoming a knight was to find Corrin again, and she's dead! So two aimless souls find each other. And... they talk.
Beruka finds herself admiring how emotive he seems to be. Silas thinks he likes that she lets herself be a bit of a downer even when other people judge her about it. The two keep chatting. Beruka doesnt really smile, and Silas doesnt really cry. But the two of them do let the other feel... better about themselves.
But Selena gets married to Odin. (Beruka gets invited.) Beruka's not sure how to feel, but she has a slightly higher sense of what people feel about her. And she knows they're talking. That they expect her to be able to adapt by now.
So. She proposes to Silas soon after. Silas accepts, just as eager to get eyes off of his back. They do not marry out of love, or out of one wanting to fix the other. They marry to help each other.
... and because beneath it all, they do have a genuine bond, even if it is an odd one. One that does grow during the marriage, after the birth of their child... even if it is not in any conventional way.
