If I play Rebuke on enemy unit at battlefield and then they react with Here to Help hidden and play Brynhildr there, does my Rebuke still go off?

Yes, your Rebuke will still resolve and return the targeted unit to its owner's hand, provided the target is still on the battlefield when Rebuke resolves.

Here is the step-by-step breakdown of how this interaction plays out:

1. Initial State (Open)

• You play Rebuke as an [Action] at a battlefield with an enemy unit. You declare that unit as the target.
• The game enters a Closed State because there is now an item on the chain.

2. The Chain

• The chain currently looks like this (from bottom to top):
  1. Rebuke (targeting the enemy unit)

3. Opponent's Reaction (Closed)

• Your opponent reveals Here to Help from hidden. Playing a hidden card has [Reaction] timing and can be played while the chain is closed.
• The opponent pays the costs and plays Brynhir Thundersong to the battlefield.
• Brynhir's triggered ability ("When you play me...") goes on the chain.

4. Resolution Order (LIFO)

The chain now resolves in reverse order (Last-In, First-Out):

1. Brynhir's ability resolves: Your opponent's ability triggers. According to the mechanics, this prevents you from playing new cards from your hand for the remainder of the turn. However, it does not counter or affect spells already on the chain.
2. Here to Help resolves: The spell finishes its instructions.
3. Rebuke resolves: Finally, your Rebuke resolves. Since the target unit is still on the board, Rebuke performs its effect and returns it to its owner's hand.

Why this happens:

• Targeting is locked: Per RiftJudge FAQ #4486 and #7586, targets are locked in when the spell is played. As long as the unit remains on the board at the battlefield, it remains a valid target.
• Brynhir's Timing: Brynhir's ability resolves before your Rebuke because it was added to the chain after your Rebuke. Even though it stops you from playing new cards, it does not stop the resolution of spells already on the chain.

This is my interpretation based on the standard LIFO chain resolution rules. No official FAQ exists for this specific combination of cards.