In a nutshell
Sentinel has been a very infamous character in the MVC series. In MVC2 he was notable for his extremely fast Hyper Sentinel Force, which was easy to combo into, as well as his super armor, great assists, and high health. All of these traits carried over into MVC3, but thanks to the new control systems this made him extremely easy to use and abuse. Even after his health was nerfed in a patch from 1,300,000 (!) to 905,000, fans still cried foul. Now, in Ultimate, he has received several changes that have reformed Sentinel into a balanced, yet brittle powerhouse. His health was nerfed slightly more to 900,000, and he lost the ability to combo into Hyper Sentinel Force as easily as before, but he still retains his agility, super armor, and sheer power. If you like these traits in a character, Sentinel may be for you.