Mergrave

Mergrave was a doctor on the planet Castrovalva.

He was a hunter also who followed the rituals of his planet. After the Doctor explained to him that Castrovalva was folding in on it self on purpose he helped the Doctor to see the Portreeve.

When he was told to investigate a smashed window on the upper floor he found the Doctor. He explained that Portreeve was in fact the Master. Mergrave sacrified his life to keep the Master trapped in a 'about to blow up' Castrovalva. (DW: Castrovalva)