Second Light has two endings; if you're rushing through, it is possible to miss the "true" ending.
The requirements are as follows:
- Clear the game. The True End only shows up on NG+. See here for saves if you're on PC.
- Get any girl to talent level 10.
- Choose this girl at the final choice.
- Be sure to have seen her final scene at her Heartscape.
- If you heard that skipping dialogue matters, as far as I have tested it does not in current versions of the game. Skip what you want.
Differences from regular end (as spoiler free as possible)
- There is a slight scene change before the final boss.
- The girl you pick has two changes at level 10 -- the first can be seen on your first run, and the second is only seen on NG+.
- The very final scene of the game changes.