Not much is known about Magdiel's life before the party met her at the Feywilde ball. Presumably she raised Valanthe, though it is unknown if he has any siblings or other parental figures at this time. She is one of the few elves the party has met who shows obvious signs of advanced age, which implies she is quite old even by elven standards.
Her relationship with Valanthe is clearly quite strained, however. Serena revealed that Magdiel is near obsessive in her desire for her son to learn the skills of a wizard, something Valanthe himself has no interest in. This is the source of tremendous friction between the two of them, as Magdiel refuses to let the topic drop no matter how little headway she makes.
She has even tried to recruit Valanthe's employees to join her crusade, starting to comment during the feast on how much the party was able to get done with their magic (though Serena cut her off before she could finish the thought) and even cornering Serena herself later in the evening. Cree was able to offend Magdiel into leaving by pranking her with a jump scare, but by that point Valanthe had already been dealing with her enough to have stepped away from the party for a break.
Magdiel's true personality isn't much known, as the party has mostly only interacted with her in a polite diplomatic setting. She certainly seems to have the formal noble mask down to a science, and for the most part has been nothing but cool and professionally polite. This facade did crack briefly when Cree snuck up behind her and shouted to spook her, and she shot the dragonborn a very venomous look which implies she either doesn't have a sense of humor, is a stickler for propriety, or knew Cree was deliberately stopping her from pestering Serena about Valanthe.