U-mobile is run by U-Next, a video streaming company and one of major MVNOs in Japan. They use NTT DoCoMo's 3G and LTE networks.
Their plans are relatively easy to understand than the other services as the only differences between 3 plans they offered is service period:7 days, 15 days, 30 days. Plus, there is no need to activation, which is usually needed before connecting to the internet.
Available to purchase at U-NEXT stores located at major airports and U-NEXT vending machines at Narita airport.