Is it possible to make 2d games ( using 2d characters ) on unity ? How much advisable would it be ?
Sure it’s possible though Unity3D doesn’t offer built-in functionnalities for that. There’s a lot of existing solutions, free or not, to develop such projects (SpriteManager is the most popular). And if you’re an experienced developer and know a thing or two about 3D, it’s not that hard to create a 2D framework from scratch with Unity3D.
Just to reinforce what Krobill said, yes, it’s completely possible (and not inadvisable).