Help with Neon Tetra?

Ihas my Neon tetra distanced itself from the rest of the others.I have a five gallon tank with only 5 Neon Tetras. Originally I had two. I will give them names so you can keep track of who is still alive. One ped away and our buddy TOTO was left. We gave him two more very young friends and after a WHILE one died. This left us with TOTO and ERNIE. TOTO being much larger than ERNIE. We then got three more roughly the same size as ERNIE. TOTO is significantly larger than the other four. I also believe that TOTO and one other are females b/c they are more round than the others. TOTO is staying in the corner not swimming with the other fish. TOTO is a warrior and has survived all of my early bad tank habits. Now that my aquarium keeping skills have improved and all the tests go ok(ammonia/nitrates/ph) why is she hiding in the corner. It is very apparent she wants nothing to do with the other 4. She does not look sick or beat up. She is just chillin in the corner. Any help is apprecdiated