I am looking for a good German word(s) for the English "(a)lone wolf". Practically I am looking for not only a simple dictionary translation, but for a secondary/metaphorical meaning as well, if it exists.
For the metaphorical meaning, the idiomatic term is
The literal translation “Einsamer Wolf” is something you could say in German as well.
"Einzelkämpfer" and the similar "Einsamer Krieger" are common; they apply to non-competetive settings but do imply that the person is actually doing something (alone), as opposed to just being alone as a general rule.
Alle sitzen in der Sonne, aber Klaus mäht den Rasen, der einsame Krieger!
Viele arbeiten lieber in Gruppen, aber Claudia ist eine Einzelkämpferin.
Wiktionary lists the (imho) nice alternative "Eigenbrötler" to "Einzelgänger". It adds a little pinch of eccentricity if so desired.
I have to add a little more explanation, in English there is also the word loner which is best translated as Einzelgänger. The loner might be someone who simply hides and has no contact to outer world. He usually avoids every conflict with others.
In contrast the additional word wolf emphasizes the fact he is brave and a real hard fighter for something even if there might be hard resistance against him or his opinion. In German the best translation is Einzelkämpfer or Einsamer Krieger or Einsamer Wolf.
If you want a somewhat heavy literary allusion, there's "Steppenwolf"