Mericas
Mericas is a hedge wizard who lives in the woods about an one day's walk from the covenant, in the direction of Winchester. His powers seem to be oriented toward warding and binding faeries.
Mericas was about 30 years old when the magi first encountered him, so he was born c. 1165.
Mericas was living in the forest, meddling in the affairs of the fae for his own (presumably nefarious) purposes, when he crossed paths with Sandre and Caballus in the summer of 1195. Mericas seemed to mistrust and fear Hermetic wizards. They told him he had to join the Order of Hermes so he accepted.
Later the same season, Godwyn and Hywel Mawr caught Mericas kidnapping a well-born mundane boy. They took the boy away from him but didn't punish the wizard.
Personality
Mericas did not know Parma Magica in 1195 so he showed hostility to the magi. In fact it would not be going too far to describe him as "reclusive" and "paranoid." He seems to value his privacy, which would be all right if not for the reasonable suspicion that he habitually breaks the Code of Hermes.
Powers
The magi of Stranggore don't know for sure what powers Mericas has. They have seen him summon sprites to defend himself against intrusion by the magi, but they've never seen him in an actual fight. Merewen has never met Mericas but she asserts that his powers must be weak because the powerful wizards of Britain joined the Order centuries ago.