-chan = suffix used for children, people younger than you, or close friends.
-san = standard Japanese name suffix, shows politeness.

Piro speaks:
hehe, oops.
i didnt even notice it (nor most others, it seems) till Patrick emailed me about it... i forgot to draw ping's ear blades in panel 2 ^^;;
i'll fix that up tonight. its not that noticeable, but its a doozy of a 'oops' :P