This is a useless criteria for public policy. PETA looks at a chimpanzee or even a fish an sees "human characteristics" (though, interestingly enough, they don't seem to see them in unborn humans). I see human characteristics in a fertilized egg. Michael Tooley doesn't see them until an infant is about 2 years-old. This doesn't help to draw an objective line at all.
If so, those trigger my emotion of empathy and I want to protect it.
This is even less reliable. Some racists have no empathy for people of other ethnic groups and, using similar logic, often think of them as "less than human". "What do you think?" is how conservatives should approach issues. Leave "How do you feel?" to liberals.
The wrong way to approach this issue is to decide how you "feel" about abortion socially and then go out and try to find criteria to justify your feelings. Think about the issue and make up your mind. Thinking about it changed my mind.
As the human characteristics strengthen, I want to increase protection and sanctions for killing the fetus.
What "human characeristics"? Which one's are important? When do you have enough of them to be protected from execution at the whim of another? This criteria raises as many questions as it answers.
As for the sense of empathy, a racist may not empathize with other races, but that's a social and individual sickness, it's not normal. You can outlaw racist acts but you can't outlaw racism. Abortion is a sign of societal sickness and though laws would motivate better behavior, they won't cure the sickness.