Sympathy = understanding others and agreeing with their feelings towards whatever the feelings are directed at. Can also be interpreted as empathy; compassion towards others.
A super-sympathist is one who cares very much about others and their situation, despite of personal opinion. I.e. those who have read/watched Junjou Romantica: Pure Romance know Misaki to be a super-sympathist, proven when he cries for Usagi's unrequited love towards Takahiro(Misaki's older brother who suspects nothing of that love), after the latter announced his marriage to another person(and girl).
Misaki is also a good example of a super-sympathist, because he has always been very considerate of others(as said by Takahiro to Usagi) and their needs due to the fact that he lost his parents in a car-accident, which he believes he was at fault for.
Misaki from the manga Junjou Romantica; Pure Romance(also an anime) is a supersympathist.