Strax

Strax was a commander in the Sontaran fleet when he encountered the Doctor. To restore the honor of his clone batch, the Doctor sentenced him to service as a nurse, constantly having to help others recover from their wounds rather than inflict them, and this role actually suited him well, despite the bluster he initially demonstrates. Strax quickly grew into a firm ally of the Doctor, a wholly individual Sontaran, unlike his clone batch brethren. Thus far, Strax appears to be an anomaly- all the other Sontarans we meet are solely focused on conquest with very little if any unique or defining characteristics.