She needs marking as her property.

A short time later, Ladydevimon walked over with a needle and a gold nipple chain, with the japanese characters for 'Ladydevimon's Slut' dangling from the links.

"A little more personal than a simple tag..." She says, taking the needle, and shoving it through the first of Hikari's nipples, drawing a small amount of blood. She then placed the first ring in the hole, the chain now dangling.

"Why are you putting this on me?" She asked worriedly.
"Why Hikari, It's simple. When I'm done with you, You'll want to declare to the Digital World who you belong to..." Ladydevimon says, "This also will be a lot more permanent than you'd expect..."
She then took a small gold cap, the Light motif dangling from it, and placed it over Hikari's pierced nipple, then, with a small hammer, **** it over it, pinning the chain into the nipple, and causing even more pain as it deformed the small piece of flesh. Hikari was in agony now.

The second nipple was done and hammer-sealed while Hikari was still trying to cope with the first one, until it was finished, and Ladydevimon's sick banner was spread across Hikari's agonized body.
"You sick bitch! Why do you think I want this on me?!?" She whimpered, to get a slap across the face.
"You, my dear, Are no longer Hikari Kamiya, Child Of Light... You are my slut, my property... If I want to shove a pole into you and watch your body twitching in pain, I will..." She says, "You are one of the most sought after things in this world... and now, you are mine..."

Hikari sobbed, not really noticing as Ladydevimon went to get the next thing she'd use on her... She already knew that it would be too late before someone would arrive for her...
She was marked, and in a way that would be even more agony to remove completely. She almost wanted to pull the fragile links off her body right now, but her nipples still burned, and it would undoubtedly tear them open right now...