A backstory needs to matter to the story; in this case it probably matters to the MC; few people fall in love with a person they know nothing about. They fall in lust, certainly, and that lust can lead to true love by compelling them to pursue the object of their lust and thus get to know them personally. But lust isn't love -- If in getting to know the object of their lust they find repellent ideas, cruelty and selfishness, disgust with the personality can defeat both love and lust.
In that sense, you may want the Rufus backstory mostly told to the MC, to create sympathies and understanding for Rufus with her, supporting the cause of her falling in love with Rufus. It becomes more convincing to the reader if they share in the MC's experience of Rufus, instead of them being told something the MC doesn't really know. And story wise, this is more efficient: If you tell the reader something the MC doesn't know, and then later Rufus needs to tell her all about it, then as an author you are repeating yourself and boring the reader, because when they read this second passage, the reader is not learning anything new at all. (Of course somebody besides Rufus could tell the MC, and Rufus not talk about it.)
There is some magic in romantic love, some undefinable qualities are necessary, but there are also some commonalities we expect to be present for true romantic love, as opposed to similar motivators like lust, or chivalrous honor, or heroism in protecting the weak. One of these commonalities is spending time together and getting to know each other, personality is an important component of love, but not so much the similar motivators listed; we can lust after a beautiful body without knowing a thing about the person inside it. Or pull such a person out of a car crash, or intervene to prevent a bully from beating them.
Romance demands a fit of two personalities, usually one with synergy, so the two together are "better" in some sense to each of them, than they are alone. That synergy is the magical part that you, as an author, can assert as feeling.
But if you want them to fall in love, they do need to get to know each other, even if much of that is off-screen, you have to make sure the reader understands they are spending time together and learning about each other. Wherever you engineer your story for these conversations to take place, that is a good time for backstories.
These "times for conversation" often occur in transitional times within the story; for example while traveling somewhere, or on a stake out, or in an office setting working on a project, or while one of them is in the hospital recovering from injury, etc. Avoid a "talking heads" scene, make it so they are doing something (or things are going on around them) that doesn't require a lot of mental attention and allows them time to have a conversation.