Why does Moxy have a doll form, and why does she seem to like to say it in so much?

The short answer for the first question is that it was part of the evolution of her character. Buuuuuut this journal ain't for short answers or it wouldn't be too interesting.

First off, Alison's Education had two prior incarnations: "Scorpio Troubleshooting Services" and "The Royal Guard". The first was sci-fi; Alison was a cyborg, and Moxy was from a race of catgirls. I then didn't like the idea of Moxy's mother having so many children (six!) and still be a fighting fit single mom (repeatedly widowed) so I then changed Mary into being an artificial life form. Since it seemed to be redundant with Alison (who was also an artificial life form) that mutated into a more supernatural stance.

This led to the Royal Guard, where some of the characters like Mary, Moxy, and Frogboy were "dolls" -- all variations on the golem idea of an animated, supernatural guardian. The Royal Guard had a humours setting which became the basis for Super Temps: The royals were a bunch of super-powered nutjobs whose purpose was to distract the attention of the populace from the real issues, and sell tabloid papers owned by the government. The Royal Guard's job was to protect the royals from the public and each other, as well as protect the public from the royals. (Still a good concept, when I think about it)

Mary was an animated "Guard Doll"; she was made from plastic. Moxy was a plush child's comfort doll, so she had special protective abilities. I dropped the Royal Guard idea (now that I think about it, I might merge it with "Golem") because the main characters didn't seem to fit right. Alison had an excellent personality for the story... but she, Moxy, and several planned cast members didn't quite seem to "fit" right when I thought about their origin stories.

While I discarded "Royal Guard" for the time being, the idea of Moxy's doll form stuck; it actually seemed to be the best explanation of her origin. I decided to not allow Mary to have biological children, because I felt her unfulfilled desire to nurture children of her own added to her character. Plus it'd mean I'd have to figure out who Moxy's father was.

So, without having biological parents, I had to figure out how Moxy came to be. The story I came up with was that Moxy was a doll Mary made for Alison when she was just a little girl. One day, Moxy mysteriously came to life and fulfilled Alison's wish to have a sister, Mary's wish to have a child of her own, and Annah's wish of someone to protect Alison.

Having no set-in-stone explanation of HOW Moxy came to life gave her an air of mystery and gave her character new depth.

When I decided that I wanted to do a comic in the college paper that actually starred a college student instead of a recent grad, Alison's Education came to be. Not wanting to separate the duo, but also not wanting to have to explain "who's the catgirl?" or set a precedent for other beast people or anime conventions, I kept Moxy in her doll form and primarily in the background.

I couldn't keep her quiet too long; Alison needed a friendly foil, one on familiar terms. She was the only one qualified.

As the comic developed, the dynamic grew and pleased me greatly. Moxy became a scene stealer because I had a habit of keeping her quiet until I needed someone to say a stinger line at the end of the strip. Hanging back until needed only added to the air of mystery to the character that separates her from anime catgirls who are traditionally cute, but not too bright.

Besides, isn't it far more catlike to just play it cool in the shadows until needing to strike?