Why is it sisua and not sisu?
Because it's not about a single and entire countable thing. In other words, he doesn't have the entire concept of sisu. He just has some of it, so it's an indefinite amount, and partitive case is used for indefinite amounts.
Is there an English words sisu? Should it be true grit?
No. It is one of those things like having "fire".
sisu ..is this an english word?