I recently read a mystery — [URL="https://www.kobo.com/us/en/ebook/the-will-and-the-deed”]The Will and the Deed[/URL], by Ellis Peters — with this exact problem: a woman dies in chapter one whose personality and life define the rest of the book. The first chapter is included in the sample and may be worth a read for you.
This was written in the 60s and the genre is different, but one big lesson I drew from this was, if you have one scene to establish a central character like this, make it a big one. In this case, the Antonia is on her deathbed. This lends weight to every interaction. In your case, I’d consider starting with some important moment in the relationship — one of those Big Relationship Arguments (are we going to move in together /get married/have kids, I got a job offer but I’d have to move, my parents need me to move home to take care of them, etc) or Big Relationship Moments (moving in together, proposal, etc) that tend to happen to everyone trying to form a partnership. Hard to match the impact of a deathbed scene, but you’d gain the frustration of something unresolved, a story that’s interrupted, that the reader will feel as strongly as the MC. And it will give your characters a chance to talk about something that’s important to them in the brief time they’ve got on the page, rather than wasting time on trivia.
Another lesson I took away: varied viewpoints. While it’s only set up in this first chapter, the people surrounding Antonia all have very different relationships with her: her oldest friend and artistic partner; her secretary of a year, who’s only known her old; her lifelong business manager and doctor; her niece, desperate for her money, and her niece’s son, forced by his mother to learn an instrument he hates purely so he can accompany the rich, old diva. The way they speak of Antonia to the eventual MC and to each other helps create her as a whole person in the reader’s mind. If you don’t already, I would put a lot of thought into other people besides your MC that the boyfriend has left behind, and what conversations they might have with her about him. What holes did he leave in the world around him that people — your MC, but also others — will need to deal with?
Finally, as Patty and others upthread have suggested, there’s physical memorabilia that can carry a great deal of weight. Pictures, gifts given by the boyfriend, things that were important to him and thus become important to others after he’s gone: these are ways to give the reader a sense that something precious has been lost to the world, and invest them in the quest to return it.