Being underweight is not cool
Why do you think being underweight is not cool?
I don't want to be too underweight, it makes me look vulnerable. I also think, but am not sure, that it is unhealthy.
I'm sorry that you feel that way about yourself. You should set some goals that you can change the way your body looks if you don't like it.
I need to build muscle.
That's a fine goal to have. No one wants to be viewed as a pushover.