I'm kind of curious, some people may have looked a little aloof and cold from the outside, and get commented on this too but deep down she/he isn't like this. She/he are probably just quiet and doesn't mingle around, don't think it's a wrong trait? How can metta help on this? And how can metta be practiced on situation like this? Thanks.
Loving Kindness, I would think.
You are asking, really, what should we do when confronted with an unpleasant person (who is unpleasant in any way, including appearance). We cannot NOT be human, and so our reactions are, and will be, whatever they will be.
However, if you relax and give freely, of your attention and time, you may find their attitude changing towards you. You will also find your reactions changing. Through that process of feedback comes learning.
To be very clear, "giving freely" doesn't mean "giving without discretion". Give only what you would give to anyone on earth. Use your discrimination to determine how much farther you are willing to go in compassion. And end the interaction if you decide it is no longer fruitful.