Acting On Instinct is the seventh and final Relationship Quest for Natsai Onai. Some time after the showdown with Harlow, you go to check on Natty, to see how she’s doing after taking a hit to save you.
Acting On Instinct will automatically begin once you finish Harlow’s Last Stand, and that quest will take you to the Hospital Wing, where you need to be to continue. Natsai Onai and Professor Onai will be right in front of you, so walk up to them to hear a conversation between the two. Once her mother walks away, speak to Natty. You two will talk about the battle against Harlow, and Natty will have a sudden revelation.
After the conversation, you will have completed Acting On Instinct, as well as the questline with Natsai Onai!