People can hide behind many layers of identities, completely obscuring how they are feeling. Many, especially teens, hide their depression for years in an effort not to "bother" others. At the same time, depression is not really something that can be truly hid. If you know someone who is showing signs of depression, talk to them - don't make the mistake of waiting until it is too late.
Hyperbole and a Half is a very popular webcomic known for being the originator of the "do all the things!" meme. The creator of the comic went through depression and wrote these two comics about her experience, also giving advice to those who have friends or loved ones with depression.