"I vow my fealty and dedication to you, Princess Elara."
-Winter's Rebellion, Chapter 1 

Tristian Blackwood is the main male protagonist of echopen's novel, Winter's Rebellion, a fantasy-romance. He is a knight hailing from Dracoria, vowing to protect Princess Elara at all costs after the subsequent fall of the kingdom due to an ambush initiated by usurpers.

And thus let's present his MBTI:

Cognitive functions:

Te (Extraverted thinking) > Ni (Introverted intuition) > Se (Extraverted sensing) > Fi (Introverted feeling)

Based on his POVs and thought process, Tristian is most likely an ENTJ, the commander. 

SUMMARY

Being one of the surviving knights of Draconia as it fell into the hands of invaders, Tristian is extremely faithful to the royal family, especially to the late king and Princess Elara, thus he is likely to sacrifice himself for her survival.

He has a charismatic and assertive personality; his leadership as he guides his party and how he interacts with other characters is prominent throughout the story. His negotiation skills are impressive, as he is able to convince other kingdoms to listen and carry out with his planned strategies in a war against the usurpers. He is definitely more goal-oriented; he does ponder but he is eager to get things done as quick as possible for the best results. By the other characters, Tristian is described as a solemn man.

EXTRAVERSION

I'm the least confident in determining if he regains his energy alone or with other people, however I will be sticking with extraversion. Throughout the early chapters, he strategize plans more effectively alongside his group of knights and the soldiers he had obtained from the kingdom he had requested for help. Not only he is verbally excellent and actively participate in discussions, he also finds comfort, strength and trust in his peers, feeling devastated if he discovers any kinds of betrayal within the group.

INTUITIVE

With his Ni (introverted intuition) being his auxillary function, Tristian prefers to think ahead. He is always contemplating on many ideas on how to take back Draconia from the Malorians, such as recruiting soldiers from other kingdoms, persuading the kings and queens to help them and strategizing backup plans in case the Malorians ambush them.

Additionally, Tristian seems to depend on his gut feeling and instincts on making decisions. Thus, he tends to make impulsive decisions and jump into baseless conclusions; this is shown when he finds out a secret that puts both Princess Elara and everyone in danger.

THINKING

There are some moments where Tristian tries to keep in touch with his emotions, such as during his private times with Princess Elara, where he shows respectable loyalty and care for her. He is willing to do what is best for both of them.

Most of the time, however, he breaks down a conflict from a factual perspective, imminent in his POV when he tries to persuade the king and queen of a neighboring kingdom to let Elara start her training for future battles. He uses evidence around him to develop proper plans and ensure the safety of the princess. Generally he maintains his temperament, but when under significant amount of stress, Tristian's sensitivity will go up the charts, thus he withdraws from the scene to cope with himself.

JUDGING

Backup plan for a backup plan, basically Plan C if Plan B fails, is generally what encompasses Tristian's thought-process. He thrives if things are completely under his control. He will breath with ease when all of his plans are structured and run as expected. He is bothered when his routine abruptly changes, and unfortunately for him, this is common in the middle of a huge war and especially in risk of being attacked unannounced. This does not mean he is completely inflexible, though, in fact he had shown signs of adaptability such as during their escape from the castle.

To conclude, Tristian's overall behavior is indicative that of an ENTJ.

Real life examples of ENTJs include Thomas Shelby from Peaky Blinders, Niklaus Mikaelson from The Vampire Diaries, Light Yagami from Death Note and Bakugo from My Hero Academia.

What are your thoughts, if you have read the book? Do let me know! :)

Other possibilities:
INTJ
ESTJ
ISTJ