In addition to being the love of Mark’s life, Sophie is another downtrodden JLB employee.
She's a bit dowdy, quite straight-laced, mildly attractive and moderately intelligent. In other words - she's attainable, and therefore the perfect girl for Mark. She did like him, although his unfailing social ineptitude and romantic constipation has ensured that she now pretty much hates him, despite being about to have his child.
All she wants now is a stable life for her baby, and for Mark to get a proper job and learn how to drive to provide it. Oh and she wants him to fuck off as well.