It looks like a Perilla (Shiso), an herb, to me. Are the leaves fragrant? Perilla is one of my favorite plants
. It will grow to about 2 ft tall. Prefers a slightly shady area (for the purplest foliage) but will grow in sun too. It often retains more green coloring in full sun. It blooms beautiful coleus-like blooms in the fall and then produces millions of seeds
. I would suggest deadheading all but one plant
so you don't have the millions to deal with when they start coming back next year. It is a re-seeding annual.