Relationships begin with a “baseline” and then evolve. You will need to establish how your characters feel about one another at the beginning of your story. Later, in as things unfold, you’ll describe the growth of these emotional relationships over the course of the story.
Both characters need not be present to establish a relationship between them. You might your readers a look at one character’s room where he keeps a score of framed pictures of the second character, his female co-worker, in a little shrine. Then, you describe the second character’s room where there is but a single picture of the first character which has been made into a dart board. It is obviously well-used due to the great quantity of dart holes. There are three darts in it as another slams in to join it, thrown by the second character.
One character might write a story about the other for a newspaper or a school report. A photo album might show two people in a series of pictures over the years. In Citizen Kane, the relationship between Kane and his wife is established by a series of vignettes over the years in which the size of their dinner table grows, moving them farther and farther away from each other.
Of course, in real life most emotional relationships are not a single melody but a rich and complex symphony. You may want to develop a different specific means of revealing each aspect of a complex emotional relationship, or you might prefer to have a single illustration that reveals the complexity all at once.