In the New Game+ chapter select screen, there are 3 statues that can be selected, each representing one of the 3 endings. Once selected, the text gives clues for how to achieve that particular ending.

The left statue for the light ending says... At the final confrontation, "I stood my ground."
The middle statue for the dark ending says... At the final confrontation, "I felt a strange kinship."
The right statue for the neutral ending says... At the final confrontation, "I was paralyzed by fear."
While not the only determinant for the endings, the most substantial contributors to the endings are the dialogue options you choose when discussing your dreams.

In Chapter 3, Talindra speaks to you about your dreams.

You have a choice of 3 options.

1. A dream of peace : SPOILER - Talindra Peace Dialogue
2. A dream of rebirth
3. A dream of death
In Chapter 6, Mithra speaks to you about your dreams.

You also have 3 options here.

1. I was trapped with a hostile being : Mithra Hostile Being Dialogue
2. I was trapped with a familiar being : (kinship is mentioned when you choose this)
3. I was trapped with a frightful being
In Chapter 7, a mysterious woman asks what you want.

Again you have 3 options.

1. To bring justice to this realm : SPOILER - Justice Dialogue
2. To start a revolution
3. To make my own destiny
In each conversation, option #1 is light, option #2 is dark, option #3 is neutral.

Choosing all 3 light options can still result in a neutral ending depending on your gameplay choices. For example, the prisoner in Chapter 3, and the boss in Chapter 6.

Be good in these situtations if you want the light ending, be evil if you want the dark ending.

The Oksae and Adamant Heart choices likely do not impact how light or dark the ending is.

You can rescue everyone in Chapter 6 and still get the dark ending.