However, it seems practically no time has past since his blonde hair was trimmed short, even his body doesn’t look like it has struggled through a battlefield, so he is probably worked up from a lack of experience.
EDIT: even his body doesn’t look like it has seen the sight of battle. (I just learnt this English phrase.)
I can’t find many example sentence for 気負う. Is に the usual particle for the person that is worked up?
Heres my take, and ill make it short since I am actually procrastinating doing my japanese studying.
So 成り立て is actually 成る with the 立て suffix. Now the 立て suffix is basically a suffix after the masu stem of a verb that means that that verb has just occurred more or less. So he just became so and so.
Now for the に:
nounになる means “to become noun”. so 騎士になる means to become a knight. If you watch MHA, you have heard ヒーローになる a lot (to become a hero). You can do the same with na-adjectives and it changes a bit for i-adjectives, but I wont get into that now.
Now for 気負う:
The japanese definition ill role with for this is 意気込む: To be enthusiastic about.
So I would roughly translate it as “Since it looks like his blond hair was just recently cut and his body hasn’t gone through battle, it seems like hes just just full of vigor/enthusiastic (about being a knight, presumably) because he just became a knight.”
In daily situations, keep an eye out for phrases like 焼きたてのパン (fresh-baked bread). An oddity of the suffix is that while it has roots in 立つ and uses its kanji (though it’s just as often written in hiragana), it’s a nominal ending and gets linked with の when it’s an adjective. (And being nominal is the same reason it’s linked with で in your example passage.)