I have a fair bit of the Researcher in my soul, so I'll have to poke around and think a bit before saying anything about the difference between empathy and sympathy -- and the relation of these to compassion, altruism, etc.
At least one significant philosopher, Evan Thompson, [http://individual.utoronto.ca/evant/]
has argued that empathy is at the very root and core of human consciousness -- that only via empathy do we become conscious
human beings to begin with! I think he's onto something important, there.
I was just thinking before beginning to write here that empathy grows, it expands, it evolves.... And via this development and maturation of empathy over time we discover, for example, that same-sex love is essentially like heterosexual love, and that people of differing so-called "races" also partake of a common human experience, and are essentially alike, etc.... So empathy allows us, over time, to include more and more of humanity, and even of other species, into our kind -- our kindred. Difference doesn't dissolve kindredness, it merely expands it! Most enemies and strangers are so very near to being friends!