I made a character and added an array modifier, painted, and animated the character. I want to import the character into unity. Do I have to apply array modifier in object mode before importing into unity?
Yes, Mahesh, you have to apply the array modifier. An array modifier creates illusions of the object that are visible in the Blender viewport, but do not really exist. Since they don't exist, when you go to export them from Blender to the game engine, they will not be exported. In order for them to be exported, they must be applied, which creates the "illusions" into "real" geometry.
Yes, and the same goes for any modifier that affects geometry that you later want to see in your game engine of choice.
When it comes to game assets, unapplied modifiers are useful to reduce the amount of work during modeling. Unapplied Mirror Modifiers on game characters will nearly halve the time in the early stages. They also might speed up Blender's viewport, depending on your model's complexity.
Subsurf and Multires Modifiers on the other hand should not necessarily be applied before export, as long as you don't want to increase your model's polycount.