Functional Order: Fi/Ne/Si/Te
Dominant Function: Fi
Frodo takes things very personally. He’s kind to everyone he meets. He identifies with Gollum so strongly that he irrationally wants to save him. Frodo is motivated through his emotions – fear, distrust, and a strong personal sense of responsibility. He’s independent of the emotions of the people around him, can be quite cruel when under the influence of the Ring, and ultimately makes a hard but good decision to leave his friends based on what is best for him (to leave Middle-earth and find peace in Valinor).
Auxilliary Function: Ne
His conviction and belief that he can succeed against all odds leads him to accept the responsibility of carrying the Ring to Mordor. Frodo senses changes in the people around him and their implications – he knows that Gandalf is more responsible for “events” in Middle-earth than he lets on, and can see through Boromir’s lies. His discernment dims the further he goes into Mordor, eventually replacing his instincts with negative fantasies. Frodo can sometimes let his imagination run away with him – such as when he believes the Fellowship blames him for Gandalf’s death. He also foresees and believes in his interpretations of the future – that the quest will claim his life and “we’ll never see the Shire again, Sam.”
Tertiary Function: Si
He becomes very distressed when he can no longer remember the Shire, or what happiness felt like. Frodo is somewhat sentimental in his collection of things. He leaves Bag End much as Bilbo left it. He is practical but also negative in the sense that he believes they will not return from Mordor.
Inferior Function: Te
But his decisions are not always rational – he continues to show kindness to, and trust, Gollum even when the creature has proven himself deceitful, believing him over Sam and insisting Sam leave; he often leaves the leadership of the Fellowship to Gandalf or Aragorn, due to not knowing where to go or what to do, but does decide to leave the Fellowship and strike out on his own for self-protection (inferior Te).
Note: This argument has been taken from Funky Mbti in Fiction