The kyaraben (cute character bento) artist in Japan does not have to go it alone, armed only with basic tools. There’s a big array of specialized tools that she (or the rare he) can use. I saw this company’s products  mentioned in several Japanese kyaraben blogs as being easy to use and innovative. And indeed they seem to be.
It has three onigiri molds with handles; a cutter that can cut out the essential Hello Kitty and friends accessories (ribbon, flower, nose and ear) from ham, cheese or any flat food; and face punches for your nori seaweed. What more could a Hello Kitty fan want?
This product, intriguingly named Dreamland, is interesting: it’s an egg former that allows the yolk to be shaped . It’s 1575 yen. I rather want one of these.
[Edit:] Bentosupply  has a post showing how this works, with photos. (thanks clover!) The eBay bento seller she got it from doesn’t have it listed at the moment unfortunately.
Or how about these mini onigiri molds to make bitesize onigiri? The yellow set  has oval, triangle, bear, star and heart shapes, and the pink set  has a mini-menagerie of elephant, cat, generic circle, dog, and bunny shapes. Each set is 980 yen.
Here’s their complete product lineup . Click on each picture to go to the product pages, which have very clear how-to photos and examples of what can be made. Note that they are not 100 yen shop level prices, so if you add shipping and other costs they’d be fairly expensive purchases. (The company doesn’t ship overseas, so you’d have to use a shipping service or a good friend in Japan - see below about shipping services.) Still, if you are a diehard kyaraben fan…
(If you are interested in buying any of these featured Japanese bento boxes or accessories and you don’t have a relative or friend in Japan to help you out, try the overseas shipping services listed on this page . Note this is not an endorsement of any of these services since I have not tried them myself, but they do get positive reviews from Just Bento/Just Hungry readers.)