I have a unit at a battlefield with Here to Help hidden. Opponent moves a unit into that battlefield and initiates a showdown. Am I still in control of the battlefield and able to reveal Here to Help in order to play another unit?

Yes, you are still in control of the battlefield, and you are able to reveal and play Here to Help as a [Reaction] during the showdown.

1. Control of the Battlefield

According to Rule 187.4.a, if you control units at a battlefield outside of combat, you maintain control of that battlefield for as long as you have units there. When your opponent moves a unit into your battlefield, the battlefield becomes Contested (Rule 187.3.a), but your control is not lost. Rule 187.4.b explicitly states: "While a Combat or Showdown is ongoing at a Battlefield, Control of that Battlefield cannot change until instructed by steps of the Combat or Showdown." Therefore, you remain the controller of that battlefield throughout the showdown.

2. Playing Here to Help as a [Reaction]

You can reveal and play Here to Help during the showdown because of how hidden cards function:

• Hidden Cards gain [Reaction]: According to Rule 811.6, a card that is hidden gains the [Reaction] keyword while facedown.
• Timing: Because it has the [Reaction] keyword, it can be played during a Closed State (when a chain is ongoing, such as during a showdown).
• Playing from Hidden: Playing a card from a hidden state is a permitted action during a showdown (Rule 811.6).

3. Playing Another Unit

Once you reveal and play Here to Help as a [Reaction], you follow the instructions on the card. If Here to Help allows you to play another unit, you may do so.

Important Note on Location: According to RiftJudge FAQ #3476, when a hidden card (like Here to Help) causes you to play a unit, you must choose to play that unit at the battlefield where the hidden card was located.

This is my interpretation based on the provided rules regarding control (Rule 187) and the mechanics of hidden cards (Rule 811.6). No official FAQ exists for this specific interaction with 'Here to Help', but the rules for hidden cards and battlefield control are clear.