So rasi's that are shirshodaya ( signs that rise head first) are considered benefic; Signs that come tail first or pristhodaya, are considered malefic - Pisces, the natural 12th house, is both pristhodaya & shirshodaya, go figure?
Rahu is the karaka for desires and compulsion, hence why we return to this earth; Rahu is karaka for incomplete karma to manifest again. My Jyotish guru suggests the Rahu too is one of mischief.
From my studies, Rahu Co-lords Kumba or Aquarius; Ketu co-lords Scorpio or Vrishika.
Rahu enjoys being in Taurus and Ketu enjoys Scorpio. Perhaps when you mention Rahu owns Kanya, you may have meant Kumbha? Kanya is a Moolatrikona location for Rahu.
Ketu's 'official duty' i.e. Moksha karaka and upasana is found in the Moksha houses, that of Scorpio, the natural 8th house, and the 12th house, Pisces; hence Ketu's Moolatrikona is considered Pisces, as recognized by Jaimini.
Some think also that Rahi is exalted in Gemini,and Ketu in Sagittarius. I can see this logic.
On Aspects or Dristi
Ketu is for moksha... with no head, there is no ego and the elimination of desires. Ketu also does not give any graha dristi - that is, it does not aspect or give a glance to another sign by itself ( only via Rasi Dristi) because it is desire-less.
Rahu has desires and hence has dristi and special Argula's assigned to it.