Ultimately all jobs come down to fit in the end. What each specific co-op employer is looking for will be specific to that employer. Some will put higher preference on grades, some on personality, some on extra curricular activities.
All you can do is look at the stated requirements which are often non-negotiable. If they want to hire people in their Junior year, they will throw out any applications from Freshman for instance. If you meet the stated requirements (as all the other candidates will as well) then it is the unstated things that determine who they pick. Ultimately everyone picks the candidate they think will do best at their particular place.
Since this is a competition, then you do what you can to maximize the factors you can control like grades or outside interests, practice to improve your personal presentation in an interview situation and keep applying until you get offered a particular job.
Note that others will be competing, too and may be a better fit at that particular place. Just because you meet the stated requirements does not guarantee you will be picked or even interviewed. It means you are not usually eliminated in the first review of the applications.