Ema is someone Phoenix helped out a few years ago when she and her sister were in a bind. She also helped him with the art of forensic investigation. He cares a lot about her and hopes to see her succeed in her scientific endeavors.
Phoenix helped keep Ron out of jail recently, as he was framed for murder and grand larceny. Ron's not a bad guy, and Phoenix knows it, but he's not the kind of person Phoenix would want around 24/7. He's slightly annoying.
Phoenix inadvertently took Robotnik's job as the political science teacher, so Robotnik is out for Phoenix's blood, or so Phoenix assumes. He is a very scary and imposing man and Phoenix tries to avoid him at all costs. When he encounters him, he tries to be as polite as possible.