Tofu is derived from soya beans, and yes you can have it during all stages of the diet.
I find it doesn't have much of a taste, but it acts like a sponge in absorbing flavours, so if you cook it with chilli, ginger, garlic, etc... it's pretty good. I used to like deep frying it before I started the diet. It tastes sooo good crispy and cooked in chilli..... yum yum