Ember Tetra Not Swimming With School?

Sometimes fish when you buy them they have different levels of boldness. This tetra may either be bold or sick.
Does he look ok other than vivid colors? Swimming funny, spots anything like that?
I'm guessing he's just a loner. Keep an eye on him for possible disease for a few days-weeks just to be sure.

 

That corner is probably the corner with the lowest flow, the fish tend to find that spot in the tank when they are stressed out.   I would just keep an eye on him and see where it goes from here, he is probably just stressed and trying to acclimate.  

For future reference just putting the bag in your tank water for 10 or 15 minutes IS NOT acclimation, you are pretty much just dumping the fish right into your tank from the bag which will cause the fish some serious stress, and may be why your fishy is acting funny.  For future reference, after you let the bag float in the tank for that 15 minutes, open the top of the bag and get cup, and pour a little tank water in the bag every 5 minutes or so, until you can't put much more.  This will ensure when you remove the fish from the bag, they are put into water that is almost identical to the water that was in the bag.

 

I have a shoal of glow light tetra and I have one that has always took himself off to a corner on his own, join up when something exciting is happening like feeding time or if the others are just being frisky..., but otherwise off he goes...and he's been like that for weeks!! If he didn't mingle at all it would worry me, but as he does I am just putting it down to how he wants to be!

 

Two more tetras are a great idea. After that you're tank is fully stocked though.
good news about the tetras. I'm glad you're successful in your tank.