I haven't read the manga but I enjoyed the anime Fruit's Basket.

I'm probably more like Kisa (the tiger) myself. And perhaps like Yuki a little, too, in his distant attitude.

You are right in that Tohru is a very unrealistic character. I don't know if someone like that could really exist in the world. Her optimism is admirable, but it's not something that's really replicable. Things in life will get you down if you are that optimistic, selfless, and accepting all the time. People would take advantage, just as you said.