So what exactly is this code?
If Geass is considered "Absolute"1 then the Code is "Absolute over Life" thus making someone immortal. This question has some answers on how it is different. the form the Code takes it unknown but we can assume that because Geass doesn't appear to have a physical and tangible form then niether does The Code
was Charles after immortality when he tried to acquire C.C.'s code? He even acquired V.V's code didn't he? So he was immortal. Why was he after C.C.?
Charles was already Immortal when stole V.V's Code so he didn't need C.C to obtain Immortality. as i explain in This Answer i suspect that the Sword of Akasha needs 2 Codes to activate
My own theory is that the Code made Charles untouchable as to activate Ragnarok originally required 2 Codes. (However after Lelouch saves C.C before Ragnarok is activated Charles is still able to activate it).
One thing to note is that in the debate of "Is Lelouch Alive", on the side of yes one theory is that Lelouch has Charles's Code (as C.C retains her Code). assuming this side one can speculate 2 things when Lelouch ordered Charles to kill himself when they were at the Sword
- Charles pretended to be controlled and die knowing he was immortal
- The Code wasn't activated until Charles died which is why when Lelouch tried to order him again we see it repelled rather than it just do nothing (eg. when Lelouch tried to use it on Kallen and the Teacher a second time early in Season 1)
1: if you look here and look at Lelouch's, Mao's, Rolo's and Marrybell's Geass their Geass powers all have "Absolute" in their names and if a Code Barer is Absolute to a Contractor's Geass (thus immune) then they could also be Absolute to Life given the power Geass has over others