Here's some backstory to this. I've wanted to write this for a good while but never got around to it. First things first: yes this is a septiplier fic. I do not ship the real people; this is purely fictional and please do not shove it towards anyone.
Secondly, some insight to what is actually in this. The main subject of this fic is DID. To explain further. DID, dissociative identity disorder, was formerly known as multiple personality disorder. This was renamed to DID in 1994.
To have DID a child must experience frequent trauma and/or abuse, a ability to dissociate at a high level from an early age, and a detachment from their primary caregivers.
The personality forms between the ages of 7-9 so DID happens before then. The child's personality splits before being formed and creates Alters (or "personalities"). This is to protect the child from the trauma. They are usually unaware this has happened until much later. More alters can form later in life as different events occur, or ones that have been around in the back of the mind make themselves known.
I hope I've cleared some things up for you all.

Seán nervously watched the screen in front of him. Mark was in the middle of an interview and no matter how many times either of them done this sort of thing there was always a lingering ball of anxiety around it.

"So Mark, tell me about your channel," the Interviewer spoke.

"Sure. It's mostly gaming but I do a few educational videos every so often with my partner."

"And what's the name of your channel for those who are wondering?"

"Markiplier." Mark laughs at that.

"And how did you come up with that?"

"Oh it's really stupid. My partner Seán actually came up with it so I can blame him."

In the other room Seán was laughing.

Mark continued. "So my name is Mark right, and as a joke I said once I was multiplied because of my DID. So Mark… Multiplier… Markiplier."

The Interviewer laughed. "Wow that is kinda stupid. Do you mind explaining why you chose to include Mark in the name if there are others?"

"Of course. In the system I'm the host. That just means I'm out the most often, which is called fronting, so that means I'm in charge of eating, showering, things like that. It also means that I kept the bodies given name and identify with how it looks the most."

"What do you mean by that?"

"Each of the Alters have their own names, ages, memories, personalities, and appearances. Of course when they're out they always look like me because that can't change but inside and to each other they're all different. Think of it this way. If we swapped bodies for a day you would still be you even though you look like me. When you think of yourself you'd think of what you look like now rather than what you look like controlling my body."

"And do you have any Alters that find it difficult when they're in your body?"

"I have Ayano who's 17 and Female. She finds it extremely hard because the body is so different from what she's used to."

Seán's attention was pulled away from Mark by a Stagehand entering the room. When he looked back the Interviewer was ending the segment and Mark was waving goodbye.

When Mark entered the room Seán hugged him.

"You did a great job. I'm so proud."

Mark smiled back. "Thanks. Wil was co-con for a bit towards the end. I'll be glad when we're home."

"Thankfully that's very soon," Seán said, grabbing his coat. "Come on let's go."

Mark grabbed his hand as they walked out the door. Interviews were stressful.