Most of the information about Jennet is revealed in the packet of letters that Arthur Kipps sorts through in the penultimate chapter (second to last), called ‘A Packet of Letters’.

Of course, we meet her as ‘the woman in black’ throughout. But Jennet almost feels like a different character – more human.

+ Jennet Humfrye is portrayed as
a victim. She is unmarried and pregnant. Her wealthy family disowns her. Her married sister, Mrs Drablow, offers to adopt the child, on condition that Jennet not see him.
+ At her wits’ end, Jennet agrees, but finds it hard to stay away. So, she starts stalking the child. Eventually, she comes to Eel Marsh House, but is still not allowed to acknowledge that she is Nathaniel’s mother.
+ Jennet decides to abduct her son from her sister. But before she can, he is drowned in an accident on the marshes in a pony and trap. She watches from the nursery window of Eel Marsh House.
+ Jennet dies ‘twelve years’ after her son, Nathanial Drablow. He drowns aged 6 years with his nurse, Rose Judd.
+ Jennet dies of ‘heart failure > this suggests she dies of a broken heart.

How does she become a ghost?
Rage, hatred and spite stop her from gaining peace in death. She is filled with despair, anger and a desire for revenge. She cannot be reconciled to God, so she walks the earth, dead, spreading suffering.
What key words are used for her character?
isolation, passionate love (for her child), anger, bitterly, despair.
Why doesn’t she just kill her sister, Mrs Drablow?
She wants her to suffer the mental torment of the hauntings.
Wouldn’t Mrs Drablow just get used to the weird ghost popping up in the corridors of Eel Marsh House eventually?
The general opinion on Crithin Gifford is that Mrs Drablow went mad. We follow Arthur Kipps as he is tormented to the edge of sanity, and ends up drivelling, face down in the mist just before Samuel Daily rescues him. We wonder, as Kipps does, how Mrs Drablow could have stood it.
Does Jennet kill Mrs Drablow’s husband? 
This is not made clear. So it’s unlikely / the husband is unimportant.
How does Jennet kill Arthur Kipps’ wife?
She spooks the horse and trap that is carrying Kipps’ wife and infant child in the park. The child is smashed to pieces against the tree and the wife’s spine is snapped. She dies soon after.
Why does the woman in black want to kill Arthur Kipps’ wife and child?
The woman in black is the personification of despair, alienation from God and bitter hate. She spreads this wherever she goes. Anyone who sees her experiences a similar suffering to the one that she did.
When Mr Jerome sees her, why doesn’t his wife get killed?
Mr Jerome does lose a child after he sees the woman in black. You may remember when we first meet Mr Jerome, that he is an odd, withdrawn, easily spooked man – exactly like Arthur Kipps in Chapter One.
Why doesn’t the woman in black just go into Crythin Gifford and kill everybody?
Because she wants people to suffer in the same way that she has suffered herself. She wants them to watch someone they love die, preferably a child.
I don’t get what is her problem. Why does she feel like other people are responsible?
She doesn’t feel they are responsible. She’s been so twisted by despair and hate that she has become Despair and Hate (personification of those forces) and now inflicts it on everyone else. The point Hill is making is that it’s not logical, but when we despair or become malicious there is no logic to the way in which hatred spreads. In some ways this is not a book about revenge. It’s about the bitterness that strikes indiscriminately: it’s about evil.
Why does Kipps refer to her sometimes as the woman in black and sometimes as Jennet Humfrye?
Hill is using ‘the woman in black’ to keep the identity of the ghost hidden. We’re not sure if it might be Mrs Drablow.
The fact she’s in black suggests mourning – or that she is the personification of grief (lamenting over someone’s death) – as if she can’t get over the death of someone close to her.
Keeping the two names lets Hill show that the woman, Jennet, has been corrupted by dark emotions into something barely human and vague – the ghost-like ‘woman’.
Does she kill anyone physically?
Probably not. She spooks the horse. Daily refers vaguely to the fact that children die after she is seen. The connection is never made obviously physical.