Each character has different events that occur while they are soulmates. Many of them end up talking to the player about past events and why they chose to participate in the Astrocitizen program.
When Deedee is chosen as a soulmate, she will talk about her previous athletic career and how it came to an end. She can even demonstrate the final move she failed to complete; the quadruple Axel Jump, which is an achievement you can earn when she and the captain complete it. She can help upgrade and craft items as well.
When Tom is chosen as a soulmate, he will ask the player if they want to get matching tattoos of each other sitting in the shuttle to symbolize their friendship. If you agree, he will start talking about his life as a vet and cleaner in the army, a leader of boy scouts, and many other jobs that are unknown to be true. At some point, he can offer the player a "special chow," which has undetected ingredients, but is actually edible and can help them both with their hunger. Occasionally, he will start to give speeches and advice on various events that happen, the text even gets in disbelief at one point, wondering if Tom could actually be telling the truth.
When Baby is chosen as a soulmate, he will ask the player if he's a liability or not, claiming his parents said that he always "strong armed himself into more trouble than he was worth." If the player tells him that he isn't a liability, he will say that the player is like a mom/dad to him. He reveals that his real parents manipulated him into becoming a criminal in the past. Even as he pretended that he was a superhero and every one else was a bad guy at the time, he knew it wasn't true. His parents ended up abandoning him in jail after he was caught participating in one of their schemes, claiming he wasn't even theirs.
When Emmet is chosen as soulmate, he will occasionally give speeches about the universe and help prevent disasters from happening in the shuttle. He can also help upgrade items, claiming he always had a knack for inventing things in the past. Eventually, the player will learn more of his previous life. The text will reveal that he was always bullied as a kid for his interest in science and he had grown to become an underappreciated genius. He decided to work as a chemistry teacher because he enjoyed teaching others about the world.
When Maegan is chosen as a soulmate, she will become a mother-figure to the player. She, like the others, can help upgrade items and help during game-impacting decisions. There are instances when she can cure the captain when he/she gets sick by telling them to drink hot water. She can also talk about her old job as a laundry store owner and how she signed up for the Astrocitizen program on a whim, not expecting to make any friends - and especially not lovers.
You cannot have more than one soulmate. If a bonding event happens with yet another character, you will be forced to choose which relationship you prefer. Though if you choose to "cheat" on your previous soulmate with the other character, you will get an instant game over the next day, with the text saying your lover killed you by throwing you out the airlock.
If you choose to stay with your soulmate, you will get a break up event with the other character. They will most likely become insubordinate because of this, but there will be no game over and your relationship with your soulmate can continue.
Whenever you end a playthrough and survive, if you have a soulmate, there would be a different text outro.