I don't find a personal assistant for Android. I've been searching for those apps and found Skyvi which wasn't that very updated like Siri app in iOS platforms and was kinda annoying! Is there any prominent personal assistant for android!?
Welcome to Android Central! What exactly are you looking for in a personal assistant app? As mentioned above, Google Now performs many Siri-like functions. See some of the available voice commands here: https://support.google.com/websearch/answer/2940021?hl=en
It also depends on the Phone - most new Samsung Galaxy phones comes with an "assistant" built into them - Google Now does more or less everything Siri does too.