In your adventures in Star Ocean: The Divine Force, you'll encounter many characters that may be of great help, especially if your stats work together well. One of them is Nina, the group's healer and arguably the best character in the game. That's a good enough reason to be on good terms with her. You'll have a chance to get to know her better during the "Full Body Check."
Should you choose 'It is what it is. Go ahead' or 'Absolutely not' in the Private Action Full Body Check in Star Ocean Divine Force
You have two options in the Full Body Check situation: It is what it is. Go ahead (meaning yes, open my head), and Absolutely not. Choosing Go ahead will deepen your friendship with Nina, and choosing Absolutely not won't increase your friendship but may worsen it. As she's a significant character in the game, it's a good idea to make Nina happy and let her cause you lots of pain by examining you. If you skip, it'll default to allowing Nina to examine you.
This Private Action becomes available after you have completed the events of Seaport of Rythal. You'll get a call from Welch for another quest back in Delryk Village. Return and if you go into the Infirmary for a Private Action event with Nina. This is optional and easy to miss, but if you do this right, you can deepen your friendship with her. She wants to examine you to find out more about your alien species. To do this, she'll need to crack your skull open.
For more Star Ocean: The Divine Force guides, check out What do you do with 'Mini-Bunny' in Star Ocean Divine Force and What items can you sell in Star Ocean Divine Force at Pro Game Guides.