Sean walked into the front door of his house after a long day of school. It had been about three months since the Fusion Wave came through and ravaged his small town outside of Cincinnati, affecting almost 90% of residents. Sean, luckily, happened to be in that 10% not affected. Although everyone else around him was not as lucky.
The Wave had come through on a night where his best friend Jack had come over to hang out and do some homework. The two were just a few weeks from graduating high school and were working on final projects together all that week at Sean's house. Jack had gone downstairs to get both of them a drink while Sean had kept working. He could hear Jack talking to Sean's parents in the living room, who were watching TV together. Sean's mom had offered to get up and get Jack get drinks when the Wave came through. Even though he didn't get affected, the intense heat from the Wave had caused Sean to pass out.
He had woken with a shock some hours later, realizing it was now dark out and Jack had not come back with the drinks. Groggily, he went downstairs to check on him when he ran into the byproduct of the Fusion Wave on the couch: Sean's mom, dad and Jack were now fused together on one body. His mom's head sat in the middle of a wide pair of shoulders, while his dad's was on the right and Jack's was on the left. The body, now with shredded and tattered clothes, was mostly Sean's mom's, but a bit adjusted. His mom before the fusion was 5'2" at best and was a bit heavyset with a very curvy frame. Now, their shared body was about 6 inches taller, with the weight from her and the absorbed male duo distributed a bit more evenly across the body. A They now had four arms: his mom's right on the shoulders, while his dad's left arm and Jack's right sat just below. Their legs were also mostly Sean's mom's, but had a bit more muscle from the new inclusions.
The three of them sat, watching the TV and listening to the news, as the anchors explained about the recent event that had changed them. As the new entity heard Sean come downstairs, they rushed to him and wrapped him in a suffocating hug. They began firing off questions, asking him if he was ok, if anything had fused with him, etc. But all the questions was a bit daunting, since all three heads said the same thing in unison.
After calming down from his freakout, Sean spent the next hour interrogating the new being, seeing exactly who was who. As it turns out, the new creation was somewhat of a hive mind, with all three minds combined into one, but was presenting with his mother's personality. All three of the head's memories were tied together, creating a very complex situation when trying to recall memories of events from different perspectives.
Since it was late, all of them had been through a lot and with school was most likely cancelled for the foreseeable future, they all went to bed, though Sean didn't sleep too well that night. Waking up the next morning, he heard his new family member downstairs cooking breakfast. As Sean groggily went downstairs, once again he was surprised to see that the three headed being from last night was now acting and dressing like his father, though still speaking in unison with all three heads and sharing the same augmented body.
It took about two weeks for Sean to realize that this new body would switch between the personalities of all three of its heads. Sometimes it would change overnight, sometimes over the course of a couple days, but eventually all three heads would "take control" of the entire body and the other heads, all of them acting as the "personality of the day."
Even though the personalities would change, the new entity's mind remained the same. They still recalled what they had done as a different personality, still considering that to be an extension of themselves, regardless of how different the personalities acted. Eventually Sean's new family member (they considered themselves his parents and best friend in one) learned how to control each head individually, still finding it more comfortable to talk collectively rather than individually. They also, depending on who was "in control" on the day, would have different fashion preferences, which made it easy for Sean to learn who was 'piloting the ship' that day.
Now back to the present day, as Sean turned the key to open the door home, he saw his family sitting on the couch, knowing that _____ was the personality of the day.