New variables will be assumed to be real; functions which work with the real and imaginary parts of variables will assume that a variable is real. For example, re returns the real part of its argument and im returns the imaginary part, and so
If you want variables to be complex by default, you can have Xcas use complex variable mode. You can set this from the CAS configuration screen. Your choice will be stored in the variable complex_variables, where a value of 0 means that Xcas will assume that variables are real and and a value of 1 means that Xcas will assume that values are complex. Your choice will be stored in the configuration file (see subsection 3.5.10), and so can also be set there.