[Mashiro]

The software you are asking for would tell you how much your computer is using but not what all the other connected devices are up to. If you want that:
- NetSpeedMonitor: https://www.filecroco.com/download-netspeedmonitor/ (for Installation on Windows 8 and above use the Windows MSI compatibility mode)
- NetBalancer: https://netbalancer.com/

NetBalancer is most likely what you are looking for. The free version is enough to figure out what is using the traffic. It would be even enough to throttle the process(es) that use all the traffic.


Software. Uhm. If the bandwidth stealer is another connected client... you are aware that in order to find out who is using all the traffic you need a vector point where all this traffic passes through and where you can see who is causing this traffic and how much? That would be your router/modem. If it has no such feature and there is no custom firmware that has this features you can forget about it. Well protected WiFi - no such thing exists. WiFi is broken by design through many different holes that probably are still open on millions of devices because the people who made them never published fixes through firmware updates.