Q: Can iphone just slave for watch?

Hi

Yes, that is possible.

However, pairing your watch to an iPhone that is connected to a cellular network would enable you to use many more features on the watch, including features and apps that rely on access to cellular data via the iPhone (Apple Watch does not have its own cellular capability).

The iPhone (which must be iPhone 5 or later) must be connected to Wi-Fi in order to set up the watch.

More information:

Set up your Apple Watch - Apple Support

Yes - you can connect the iPhone to Wi-Fi during set up.

See step three in the following support article:

• "3. Get online and choose Location Services: Tap a Wi-Fi network to connect. Or if you're setting up an iPhone or iPad with active cell service, you can tap Use Cellular Connection."
• Get started with iPhone, iPad, and iPod touch - Apple Support

Good luck!

If you would like to suggest that Apple considers enabling Apple Watch to be paired with iPad Pro, you can do so here:

https://www.apple.com/feedback/watch.html

Trying to use the watch paired with a phone that has no cellular service will cripple it. Don't bother. The watch can perform some functions when connected to WiFi, but it can only connect to a WiFi network that the phone it's paired to has been joined to.

The Apple watch is an extension of the iPhone, not a stand-alone device.