Maybe he saw himself like a stray cat, because he is use to knowing what its like to be on your own and having to fight to get what you want in life, and also because he at the time before he met Amu-chan could only rely on himself and trust only himself. So maybe thats why his Shugo Chara Yoru is a cat, because maybe thats how he sees himself.

...cool!

i really thinck that it was hes destiny couse think of this if he wasnt a cat he will never have climbed the building on ep 1 and he will never have met amu and amu is a big part of his life