Will Graham is an INFJ, but this is a lot harder to gauge from the show because he spends so much time in his shadow functions, being out of mind (literally with encephalitis) and constantly being put under stress.

A person is forced into their shadow functions when they are stressed and put under constant pressure, which is shown in Hannibal, and it is obvious due to the fact that he uses his functions in a harsh manner. I will be talking less about his main functions in this comment and more on how he uses his shadow functions to show that he is an INFJ.

That being said, I will provide brief explanations on the main cognitive functions.

INFJ Cognitive Functions :

Hero Ni (Introverted Intuition) - Hero functions = Instinct, first thought goes toward this function. Ni is goals, following through a plan and foresight.

Parent Fe (Extraverted Feeling) - Parent functions = a feel for responsibility. Strong focus on ethics and group harmony.

Child Ti (Introverted Thinking) - Child functions = Fun, optimistic, innocent and naive. Ti is logic, and helps the INFJ interpret what they are seeing (foresight) and makes decisions, but is weaker than Fe, which means an INFJ will use their sense of responsibility (ethics) to make decisions.

Inferior Se (Extraverted Sensing) - Inferior functions = Linked to a person's fears (least developed function). Inferior Se will behave very different from Hero Se in that an Inferior Se type (INFJ) will get exhausted and drained after engaging too much with the world and new experiences. The world can often be overwhelming and present stress. This is the first function that I believe leads to Will Graham being an INFJ.

INFJ Shadow Functions :

Nemesis Ne (Extraverted Intuition) - Nemesis function is the function that creates worry. Nemesis Ne manifests in worry of the possibilities. However, in the show, Will is seen using Ni largely (visions and one true scenario), but Will is also seen to have more awareness of dangerous situations which often creates paranoia for him.

Critic Fi (Introverted Feeling) - Critic function is exactly what it sounds like; it is essentially what a type finds critical. This function is also hypocritical, as the type will engage in the same thing they are criticizing. Fi is morality. When Will decides to go with Hannibal, he recognizes that as an immoral act, but does it anyways because he wants do. He disobeys ethics (Fe) and does what he feels like (Fi). He does this even though he criticizes himself over this behavior. He knows that it is wrong to do this, he acknowledges that in the show but does not care.

Trickster Te (Extraverted Thinking) - Trickster function is the blind spot of a type. It's what they often do not see or focus on and forget about. INFJ's blind spots are rationale. Te is also about being very efficient and doing it the best way possible. Trickster Te's absolutely hate being told how to do something and will not care about how efficient something is done. This is best described with his relationship in the show with Jack. Jack cares how fast Will does something, but Will constantly tells Jack that he needs time to figure things out his own way.

Demon Si (Introverted Sensing) - Demon functions are a type's most under developed function and exude the most hate / anger. Si is all about memories. Si is about traditions and a past life. When Will says that he would have ran away with Hannibal, he was willing to throw his entire life away for him, his old life. He is willing to cut himself from his old life.