Intensity is the uncalibrated measure of engergy which returns to the sensor. It is the squared measure of amplitude (A*A = I). It consists of the real (i) and imaginary (q) part of the complex radar signal but intensity itself is no longer complex. Here are some good explanations of that: https://earth.esa.int/handbooks/asar/CNTR5-2.html#eph.asar.gloss.radsar:COMPLEX_NUM
About coherence, please see here: Some explanations about concepts of fringes of interferogram and coherence