The retainer feature is actually working very well now (except for the buggy part).

You can assign a retainer only to a character who has a higher rank. Once assigned, the retainer will rack up ranks and influence same as the teacher (sometimes faster if the retainer has appropriate traits, ancillaries).

It is very useful for situations when you have young family members around, but do not want to raise fresh armies just for them.

Once the retainer reaches the same rank as the teacher he will abandon the retainer position. You can cancel that assignment manually at an earlier time though.