Hikari, Twilight Guardian — ruling 1

You may choose not to exile Hikari.

Hikari, Twilight Guardian — ruling 2

If you exile Hikari, it leaves the battlefield before the spell that triggered it resolves its ability, so it will be unaffected by that spell.

Hikari, Twilight Guardian : règles, fonctionnement et interactions (MTG)

Hikari, Twilight Guardian is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Whenever you cast a Spirit or Arcane spell, you may exile Hikari. If you do, return it to the battlefield under its owner's control at the beginning of the next end step.

How it works : Hikari, Twilight Guardian

Original English text — Wizards of the Coast / Scryfall

#1December 1, 2004· Wizards of the Coast
You may choose not to exile Hikari.
No other card is named in this ruling.
#2December 1, 2004· Wizards of the Coast
If you exile Hikari, it leaves the battlefield before the spell that triggered it resolves its ability, so it will be unaffected by that spell.
No other card is named in this ruling.