The two true JRPGs currently available on Windows Phone are
Spectral Souls and
Final Fantasy.
Both are expensive, but both are essentially complete console games featuring a massive amount of content. The big positive is that you're not going to encounter annoying in-app purchases. Unfortunately, as typical of Japanese developers, instead of embracing the touchscreen interface they overlay a control pad.
The other game I strongly recommend is
Doom & Destiny. It's inspired by classic JRPGs and features a lot of humor. It also offers a huge amount of content minus the in-app purchases. Be prepared to spend upwards of 30 hours on this game if you seek out every last quest.