Janneke's father Robert Swann is an English muggleborn wizard and her mother Monique is a Dutch half-blood witch. Robert went to Hogwarts while Monique went to Beauxbatons, and they met when Monique traveled to England. They fell in love and Monique stayed in England with Robert, having three children: Dorian, Vincent and Janneke. Being the only daughter, Janneke ended up taking mostly Dutch customs from her mother. Dorian had gone to Hogwarts when he was of age, but Vincent wanted to go to Beauxbatons like his mother had. Despite following in her mother's footsteps culture-wise, Janneke was more interested in going to Hogwarts.

Janneke is an introverted girl, but is quite friendly to those who come up to her. She loves to read fiction and also listen to stories told by peers. Cultures both Muggle and wizard fascinate her and she wants to learn all about them. Once befriended, she is extremely loyal, possibly to a fault - if her friends asked her to be rebellious and possibly break rules with them, she will agree with very little hesitation. When in trouble, she will stick up for her friends and be willing to take the blame. This can cause her to be a pushover to those who like to manipulate, which causes her a lot of problems as well.

She has a slight speech impediment, causing her to stutter and repeat words a lot, as well as mental issues that cause her to have difficulty reading at times.