Authentic: Wise: Stoic: Empathic and not Psychopathic or Antisocial...
Please allow me to explain in a rather convoluted way - tis difficult to do it in any other manner.
The teenager's behaviour speaks volumes about his or her value system and a sense of stoicism embedded in the practice of engaging events in an authentic manner. The concept of living authentically is explained more fully on sites such as Psych Central and others I can provide if interested]. Stoicism encompasses a philosophical commitment to the use of logic, ethical behaviours and physics is thrown in there for good measure - again others explain it more eloquently than I can here, see Internet Encyclopaedia of Philosophy.
Somewhat more superficially, specially for youngsters its about making the choice to remain true to their inner being by resting the pressure to 'play the game of keeping up with social mores and what other's think'. The person described above is an individual who has considered well from an objective yet most likely profoundly empathic stance; the intricacies and difficulties that one encounters during the course of a lifetime.
This, from my reading of the question, does not represent a psychopathic or antisocial individual whose ability to engage in such a manner with others is dominated by less than altruistic motive. My paper Monster or Clown: Bad or Crazy, speaks to this last comment as well as what is written in the next paragraph
A casual observer of an incident such as the scene where "this character stands up to them and gets them to back off" might, not 'get' the difference between the behaviours of a psychopathic individual [even if apparently assisting another in the face of danger etc] and the behaviours of the individual you have described - more specifically the driving motivation to engage in the event. Such observations can be misinterpreted often leading to incorrect and inappropriate judgements.
Neither the psychopath or the authentic individual will be particularly concerned by such external things as being judged or observed etc. However, the one who lives in a field of self awareness and quiet reflection and acute observation is more likely to consider the opinions of the other who is threatening... It is in this stance of empathic engagement with the attacker as well as the victim that facilitates a greater potential of having someone who has threatened 'back off'.
One does not get to this state of mind/body/soul connection in the ordinariness of growing up.
There is no word really that encompasses the layers of character about which you speak, particularly when the individual is so young.
Some further comments to clarify my use of the words Stoic, Empathic & Wise:
An individual who engages in the world from a profound sense of centredness will behave as such across environments and situations. There is nothing that will sway him/her from their well considered and objective opinion/stance
I might add here that such individuals can gain their wisdom from a deep and often painfully empathetic nature, whereby the ability to understand the suffering of other beings- often at a visceral level can be a driving factor in their stance.
However, - and again the stoic and authentic terms come to the fore. No matter what is thrown at such an individual by those who bully, intimidate and otherwise impose their often ill conceived values on others from a fear based or emotive stance; there will be no reaction! There will however be a strong, logical and even if in a spur of the moment encounter; an unwaveringly calm and collected response that cannot be argued with by any 'rational and caring individual', much less an angry and irrational mob.
I would consider such an individual as authentic, perhaps with wisdom beyond his/her years, which from my experience augers well for a meaningful life even if events are not always positive. Such individuals engage in the myriad experiences, good, bad or indifferent that they encounter with the same centredness and inner purpose that can not help but be noticed by those around them.
Boring if you happen to be the type to enjoy the bi polar like roller coaster lifestyle many of us fall into. Matter of perspective I guess...