When someone has sisu (s)he does not give up when faced with challenges, for example. It's a tricky word to explain as it doesn't have just a single, set-in-stone definition.
In Finnish, do all adjectives precede the noun?
Yes, although in poetry you might see it the other way around.
Except sometimes participles (verbal adjectives) follow their nouns. As in 'jotain syötävää'. However, I don't know the rules governing when participles precede and follow.
I thought "Hän" can be skipped and one can just use 'on' to imply Hän on" or am I incorrect?
The personal pronouns hän (s/he) and he (they) are usually unomittable - otherwise, minä, sinä, me, te can be dropped.
me when i type: Hän on nainen sisukas
i hate u finnish :((