Would I need an aligner to fix one crooked tooth on my bottom front teeth? Could a retainer move one tooth? (photos)

Aside from an Invisalign Express treatment to address the lower arch crowding, It may be possible to address the mild crowding with interproximal reduction (carefully reshaping the sides of your teeth to make room for the tooth that is crowded out from the rest of your teeth) and then securing the result with a retainer. Please make an appointment with a Board Certified Orthodontist to review all of your options. 

There are many ways to treat a single tooth that is out of alignment. A consultation with an Orthodontist or a General dentist that does Orthodontics can help you go over some of your options. Although you only see one tooth out of alignment your dentist may see more.  Having your bite evaluated will help you achieve functional stability as well as a good esthetic outcome.