I think Heather, Justin, DJ, and Lindsay had the most development over the series.
Heather-Started off witchy, but has shown signs of change.
Justin-Started off no character at all, but look at him in TDA. No explanations needed.
DJ-Not much development in TDI, but had good development in the storyline of his alliance in TDA.
Lindsay-Started off your typical blonde character, but episodes like That's Off the chain and The Sand Which Project showed signs she wasn't that dumb.
What do you guys think?