To begin with, there's Molly, who is just awesome in the sense that she has so many different sides: she can be very mean and rough, yet at the same time, she can be nice, giving advice to other kids, working with them on things (The Agent of Change), being kind to her brother...there's just a million different facets to her personality.
And then we have Rattles, who doesn't show as much diversity as Molly, but still a lot. Sure, Rattles never shows off his nice side that much, but its still evidently there. There's just something really likable about him. Same with Slink.