What would you call someone who knows that they've already won before they've even started? It's bordering on overly confident, but it's obvious that there is no way for them to lose since they believe that dying isn't a loss? I'm trying to describe a character that has a bomb inside her rib cage. It can only be activated through death. She doesn't care wether she lives or dies in battle because she knows that she will win. Ex. "Mary doesn't care if she lives or dies since there is nothing to lose; in the end she will win all her battles. She is [...]."
I would call such a person invincible.
invincible: incapable of being conquered, overcome, or subdued
Like your character, a person who is invincible cannot lose: even if they "lose", they win. Kind of like, when flipping a coin, I say, "Heads, I win. Tails, you lose."
If you're looking for an adjective,
self-assured, "having or showing confidence in yourself and your abilities"
approaches the meaning described.
A phrase would make it clearer:
self-assured of victory.
It can also be expressed as:
believe/consider/think oneself the sure victor.
Several words come to mind. Without context or an example it's hard to know what you're looking for... Consider
temerarious, from temerity [MWD]
the quality of being confident and unafraid of danger or punishment especially in a way that seems rude or foolish
venturesomely bold in action or thought
acting or done in a very open and shocking way without shame or embarrassment
confident and aggressive in usually a rude or unpleasant way
Your protagonist could be fatalistic:
believing that you cannot prevent things from happening, especially bad things
— Macmillan Dictionary
Relating to or characteristic of the belief that all events are predetermined and therefore inevitable.
— Oxford Dictionaries
although that clearly doesn’t capture the full scope of your request.