Things about Magari that are canon:
They are NOT a dadster, but in their canon they have cared for a fellow Fragment's family due to being such good friends with him.
Their typical voice is that of a 50s radio announcer. They seldom drop that act for anybody, preferring to make people feel like they're in the spotlight and not them.
They have a human form, but they only take it when they feel that they would either not be accepted as a skeleton or the person that they're with prefers the human form. Otherwise, you're damn well getting to see the skeleton. ( Human form: https://media.discordapp.net/attachments/862514128165273631/889607658028560445/775199_Zhgr0XXq.png )
They used to date a Human Mage before the War between Monsters and Humans, and when he was wrongly attacked by other Monster's, they went on a rampage and slaughtered both parts of their side and the other side. Similarly, that boyfriend fully died Underground and became known as Napstablook, and later, Napstaton. Because of this, if their timeline had a severe reason to fear them, and later on down the line, when their Queen Toriel declared war on the Humans again, they... took matters into their own hands to stop it.
Their Mettablook was the Human sister of their boyfriend who wanted to still be a part of their family despite the war. She was the only Human in the Underground until she died and the power from the barrier turned her into Mettablook, who struggled with his identity for a while before settling on the fact that he wanted to be referred to as a guy.
Neither of the Blooks remember their relationship to Magari, but Napstaton did still end up dating Magari again. He was treated as a different person with a different set of hobbies and likes and traits.
Despite the fact that they came from an Underswap timeline, meaning they were slotted into the River Guardian position in their timeline, they helped build the Core with their best friend Hera (the royal scientist) after she landed the job from making Napstaton his new robot body.
The experiments on Human Souls never happened in that timeline, because Magari shut them down. Hera had tried to do experiments with Frisk (the royal Human) but Magari made sure to shut that shit down before the King and Queen found out. The two of them still collaborated on Soul research, but being much more careful about it. And the intent was never to break the barrier, but instead prolong life to Monsters and make life Underground much more tolerable by giving them something to hope for.
Similarly, the Amalgamates never happened. Because in their timeline, the experiments for preventing a Monster from falling down actually WORKED, with Hera helming it and getting advice from her fellow workers and her newest assistant. Undyne was always a sweetheart but if she was handling it, the Amalgamates probably would have happened. There are ... other horrors lurking in the Lab however. Magari knows about them but will take that information to their grave.
Magari is a lot smarter than they let on to anybody initially. They studied the makings of shadow golems with their boyfriend above ground, underneath the watchful eye of their secret coven, meant for giving people with illnesses, or disabilities, or something along those lines, a way to access the outside world without having to strain themselves too much. (This research got cut off by the war, but there were some people who got a hold of this magic outside the coven due to them knowing the coven personally.)
Though they did not have the two skeleton brothers themselves, they did check up on them from time to time. They never got too close, but occasionally, Papyrus would visit the shop they owned in Waterfall for either handmade kreteks or cigarettes. Despite their hesitation to do so, he was always a regular customer and never came in too frequently for the same thing.
They gained most of their experience in cooking and brewing from the experiences they had on the Surface. Their boyfriend couldn't cook even if the world depended on it, but his sister was a savant. Over time, they learned how to make more exotic dishes with what limited resources they had, and even figured out how to prepare a Soul for consumption.