Galaxy S6 can become a WiFi Repater by using an application called "VPN Hotspot". See the screenshot below.

Fig1. VPN Hotspot is around in the middle with a green icon.

Now WiFi Repater and USB tether have been turened on. It works pretty well, although it runs a bit slow. Note you have to turn on rndis0 monitering for USB tether working at the same time with WiFi Repeter.

Fig2. Repeater mode on the top. (Root priviledge required)

The Password for the repeater is automatically generated and it changes every time, so that you have to type this random number everytime, which is stressful. You can not change this settting, because this is required by Android OS.

Fig3. SSID and its password change every time, which is stressful.

Note the fixed 2414MHz band. It is for my lovely iPhone4, which has only 2.4Ghz band support.

Note on Fig2: It has a function of usb and bluetooth tehtering each, which is amazing.

Fig4. USB tethering.