When factoring polynomials, Xcas won’t introduce complex numbers if they aren’t already being used. For example,
factor(x^2 + 2)
will simply return
x^2 + 2
but if an expression already involves complex numbers then Xcas will use them;
factor(i*x^2 + 2*i)
will return
(x - i*sqrt(2))*(i*x - sqrt(2))
Xcas also has ways of finding complex roots even when complex numbers are not present; for example, the command cfactor will factor over the complex numbers
cfactor(x^2 + 2)
will return
(x - i*sqrt(2))*(x + i*sqrt(2))
If you want Xcas to use complex numbers by default, you can turn on complex mode. In complex mode,
factor(x^2 + 2)
will return
(x - i*sqrt(2))*(x + i*sqrt(2))
You can turn on complex mode from the CAS configuration screen. This mode is determined by the value of complex_mode; if this is 1 then complex mode is on, if this variable is 0 then complex mode is off. This option will be stored in the configuration file (see subsection 3.5.10), and so can also be set there.