What makes human resource a bridge between employee and employer?

Honesty of the work and dedication towards the work and good cooperation.

HR is the consultant between the two ; It's sort of an institution for employees and employer within the organisation to reduce the communication gap, to increase the productivity ( helping the employees to identify what do to make the org.
a better place) and advocating for employees rights even if the the employees are not aware of all their rights.

its physical existence as a medium that transforms a candidate to  an employee

The HR professionals are constantly in touch with top management and they are the one who hire, co-manage and develop them and they could retain them for a long duration with organization and to do so they could develop such polices and promote healthy culture which attract and retain talented individuals with firm and this is possible only when they present the best and pragmatic solutions to top management.

feedback from the team leader to the HR and HR to the employer.

HR works as a binding force in any organisation between employer & employee.
It reduces the communication gap and tries to bring employer-employee consensus ad idem.

Relationship between the two...

it is depend on employee