How can I show field from profile2 instead of username depends on user's role?
You can show an alternative name using the Real Name module.

However, you can't show one field for employers and a different field for applicants with Real Name out of the box.

What I would do is reuse the field_real_name in profile_employer and change the label to the company name; this will allow you to use field_real_name as a token for both employers and applicants.

In Drupal, you generally want to reuse fields across content types instead of making new fields if the uses are similar. Doing so makes it much easier to use modules like Real Name, Views, and Rules to manage your entire site.