I have never come across a rat needing to live alone. I do/did everything possible to avoid that at all cost.
Rats learn from other rats, they get comfort, security, grooming, love, fun from other rats. Even the scuffles are good... it's all part of living in a colony which is what rats do.
But some pet rats are exposed to all sorts of things in their short lives which shape their behavior and most times need patience, love and attention as well as a medical intervention to help them overcome what has been ingrained in their behavior.
Some rats are born aggressive... thanks to the irresponsible breeders. Most of the time a neuter sets them straight and puts them back on track to a full and healthy life with buddies.
But some rats need work... rats that are taken away from mom too early don't have that social teachings from mom from the beginning. Without care and love from mom and humans early on, this changes a rat. Again, most times a neuter, even a spay can help.
But don't mistake his/her unfortunate beginnings in life as a need or want to be alone. They need help to overcome their stress and anxiety with other rats. But when they do... it's a beautiful thing to watch.
I've had several lone rats.. I worked with them and found the perfect companion... for Radar, I gave him Isabelle. She taught him what it was to be a rat... Radar came out of his shell. Isabelle made him ready for rat buddies and he sure had fun when he was finally put in a colony of 12. Radar was also neutered which helped tremendously.
I have had females that are more aggressive and mischievous than most. For example Julia. She was an angry little thing... she had to be first, she had to eat first... until one of the baby males became older and became alpha. Julia mellowed out completely. She needed a good strong Alpha to take over and reign over the colony so that she could just sit back and feel secure with the right alpha.
Each rat have issues and you need to work with those issues to find the true rat in them... the true rat lives in a happy, healthy colony.