I used a low ISO, I think it was around 400
Oh... Portraits, as a rule of thumb, need to be shot at 100 or lower.
That's a stupid rule. It's rules like that that encourage people to not actually learn to understand and make their own decisions. Shoot portraits at whatever ISO is right for the shot. That's the only rule.
As for the shots, some are pretty underexposed, other are massively underexposed. The posing looks pretty stiff and a lot of the crop are awkward. Also, the lighting is really flat and the focus seems to be off in a few. Good effort, keep practicing!