Arbitrary Headcount Limit: You are only limited to 4 party members when traversing "Unknown Locations." (not to mention Nathan is the only party member that can't be removed)

And I must Scream: The "Thralls/Imperfect Reflections" are actually Nathan from an alternate timeline that committed suicide only to later be resurrected and used as puppets in service to The Big Bad as their minds watch in horror, unable to do anything and wishing for death.

The Atoner: Nathan

Ax-Crazy: Asterik

Badass Boast: When a party member gains a "Vigor" from reaching 100% Strain, they tend to do these.

Battle in the Center of the Mind: Fighting against the denizens of the Underworld.

Brainwashed and Crazy: Nathan in the Genocide route being controlled by The Big Bad.

Bookends: The Neutral ending.

Enemy within: The Darkness Within.

Deader than Dead: What happens to your party members when the Big Bad uses his "Oblivion" ability. (except for Nathan due to the Big Bad wanting to let him watch as his friends are picked off one by one)

Face Death with Dignity: The reaction of Nathan's compatriots when the Big Bad uses his "Oblivion" ability. (see "Deader than Dead" above)

Eldritch Abomination: The Big Bad.

Eleventh Hour Superpower: Things seem bleak, all is lost, the Darkness Within has killed and devoured all his friends, yet his WILLPOWER refuses to be extinguished. If he can't fight for his friends and family, he can fight for himself!

Everything Trying to Kill You: The denizens of the Underworld could be described as such.

God of Evil: The Big Bad in the Pacifist route.

Go Mad from the Revelation: Nathan's fate in the Neutral ending.

Driven to Suicide: In the Neutral ending, after realizing the possible danger he will become to those he cares about, he commits suicide in a futile attempt to rid the world of the evil within himself, only for that same evil to resurrect him into a Thrall. (see "And I must Scream" above)

He who fights monsters: Nathan. (can be taken literally in the Genocide route)

Hell is that Noise: The Darkness Within's laugh.

Love Interests: Many of the female characters can become this if they reach 100% Affection.

Love Redeems: And it's required to further the Pacifist route.

My god, what have I done?: Nathan's reaction in the Genocide ending.

Sealed inside a person shaped can: Nathan and The Big Bad in the Pacifist route.

Title Drop: The intro.

Tomato in the Mirror: Throughout the story, many of the monsters underground talk of this strange figure going around and killing monsters left and right, some even accuse Nathan of being the figure. Nathan searches the vast underground trying to find this individual and clear his name, but in the end the strange figure turns out to be himself from a previous timeline.