Spirit companion is a conjuration. If you check the conjuration keyword defintion (as is presented in the appendix of PHB2) it says something like ... (using my own words since I don't have the book handy)
Most conjurations can't be attacked, but if it can (*note the spirit companion can) then it uses your own defenses. Unless it specifically targets conjurations, then only the initial damage affects the conjuration, stuff like ongoing damage and conditions do not.
(again, i'm paraphrasing from memory since i don't have the books handy)
In contrast, the summoning keyword specifically has a clause that says "temporary bonuses or penalties to your defenses to not adjust the summoned creatures defenses."
Since the conjuration does not have that same clause, I would suspect that even temporary modifiers to your defenses are applicable to the conjuration (whereas they would apply to a summons). I.e. whatever your defenses are (even with temporary bonuses/penalties) at the time the conjuration is attacked is exactly what the spirit companion's defenses are.
I don't know what astral majesty or astral resistance are off the top of my head. But i suspect the later offers resistance? I don't know if resistance counts as a defense - so the answer to that would determine whether or not the spirit companion also has it. (my gut reaction is to say no to resistances/immunities/vulnerabilities since defenses always seems to refer just to AC/Fort/Ref/Will)