Yaneura no Rudger

Similarities: 1. Both story focused on a young girl 2. They have a family issue, which affects their personalities 3. Due to that reason, they try to escape from reality with their own way (one by transforming into a cat, the other by exploring the world of imagination) 4. They both have a guy who they feel close to (one as a crush, the other as an imaginary friend) 5. The main characters are being targeted by a villain who tries to steal something 6. Supernatural/fantasy elements Difference: A Whisker Away was about a middle school girl, and it had romance. While Imaginary was about a younger girl, and her imaginary friend. 

It's a kind of cliché to liken any good children's movie with Spirited Away, but I think this one deserves it. It has a very coherent inner logic, a good flow and beautiful animation. Not to mention a very touching story.