My satellite inclination is almost zero, but the latitude that I got was almost 0.1. In my mind, the latitude is should be near zero when the inclination is zero, but as I explain above, I got different. Is there something that I miss?
First, 0.1° latitude is near zero.
Second, latitude should be expressed in Earth-fixed frame, which uses the true equator, you might be using a Mean-Equator, Mean-Equinox frame (such as J2000), which also creates a difference between frames.
Third, it seems like you are not using a two-body propagator, such that perturbations are present and do cause effects like the one you are seeing in the latitude plot. I'd expect your osculating inclination to oscillate by a similar amount due to earth's gravity anomalies in LEO (not so sure about GEO, though).