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: 
You can use [hook\_node\_view\_alter](https://api.drupal.org/api/drupal/modules%21node%21node.api.php/function/hook_node_view_alter/7) to alter the build array. This is a [render array](https://www.drupal.org/node/930760) so this should be an easy way to update the data.
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.