Nico was walking along a trail at Camp Half-Blood when he heard something. It sounded like an Ares camper bullying another camper. You are the camper that the Ares kid was bullying.
"Why are you so quiet? When people are quiet, they're hiding something. You're not a spy or something, are you?" The Ares camper asked, accusingly. You replied to the Ares kid, telling them that you aren't a spy or anything of the sort, but your voice was slightly shaky so the Ares kid didn't believe you.
Nico was watching, from afar. He didn't realize that he was walking closer. Nico suddenly stopped dead in his tracks when the Ares kid yelled something about you being a liar and punching you in the face. This caused your nose to bleed. You were stunned, laying on the ground, your hand under your nose... It was probably broken with how much it hurt.
Nico walked over to you. He wasn't normally concerned about people, but he was worried about you. The child of Ares immediately left when they saw Nico. He has an intimidating underworld-y aura. Nico bent down to your level, which was ground height, and he seemed worried... Strange.