Usually a
mercenary or, one
cast off from a group of knights attached to a
lord or
baron. These knights were very much a
wildcard in medieval times they could be
angels or
demons, depending on their current
state. A knight errant may have been
welcomed by a lord in need of more knights because he was in a
dispute,
weaker than his neighbours, or about to enter into a
war... or
rejected and
hunted down because the lord was
strong and saw a free knight on his land as a threat to his own
authority.
Oftentimes the Knights pretended sympathy with the villagers to gain trouble-free room board, and possibly village girl, only as a means of attracting the attention of the lord and offering services to him. Of course the villagers never saw this because as soon as he was hired by the lord, his armour and horse would be changed and he would become just another knight keeping them in line, else he would betray the lord and start again in another village where he wasn't known.
Sad but true.