Question: I would like to show a default value if the user doesn't fill in a field in the user profile.

For example: 

I have a Hair type field. If the user fills it in, i would like the following output:

**Hair type: long.**

It is already working. If the user doesn't fill in the field, i would like the following output:

**Hair type: Not specified.**

I use Display Suite to display the user profile. I don't want to set a default value to the field, because i just want to show the Not specified value on the user profile page.

Which contrib modules or hooks can i use to solve this problem?

Answer: 
To add default value to User Profile field. Go to admin/config/people/account settings/manage fields and edit that particular field. When you on the edit form of that field, there is a field to set `Default Value`. You can add any default value that you want in that field.
Sorry, this sentence doesn't make sense to me:
"I use Display Suite to display the user profile. I don't want to set a default value to the field, because i just want to show the Not specified value on the user profile page."

I think Krishna Mohan answered your question.