They should be employees with good people skills.
They should not be line manager
Fairly recent recruits are more likely to empathise with what the new employee is going through.
So someone who joined the year before might be appropriate.
The research into mentoring schemes suggests that informal schemes work at least as well, if not better, than formal schemes. (Clutterbuck is the expect on mentoring)
The benefit will come if there is a rapport between the pair. If not then questions will not be asked, people will be too busy and the relationship will not be of value.
Col
colbrown.co.uk