Whoa, you captured the style amazingly well. Kudos.
For Noktra, judging by her fingers, the palm seem to be facing her (with her knuckles facing outward) so it looks like her elbow is bending slightly the opposite direction? And her shoulders seems so stiff, relax them a bit. A female's shoulders aren't broad so think if a hanger.
For Rivyn, her breasts are a tad low unless she's maturing (you know, in the 40s :P). Since she seems to be leaning on the wall, there should be little shadow from the head and the size gradually increase as they reach the floor.
I love Invader Zim too. Keep it up. ; )