How do you fall in the friends category?

There are a lot of factors, but ultimately if you do not meet the interests and/or conditions the other person has in that of a mate or an enemy, then you'll go into the category of a friend.

Looks play a factor but not the ultimate factor for many people. I know of people who have said that the person they were attracted to (or even married) was not that good-looking.

Of course, this can be changed as the person's interest changes, and/or as you show disinterest in that person of anything other than a friend. Rarely though does a friendship further development into anything more once your friend casts that role as your 'only' role in their lives.

Jam it back in, in the dark.