Hornet

Information
Hornet is an ally character who is first encountered in Greenpath.

Origin (spoiler warning)
Hornet is the child of Herrah the Beast, though whether or not she is biologically her daughter is unclear. Herrah traded her allegiance (she previously did not accept the White King) and took on the role of a Dreamer in exchange for a child.

Hornet herself says that she shares the source of her power with the player character, though she lacks the emptiness which makes them a suitable vessel.

Journal Description
"Skilled protector of Hallownest's ruins. Wields a needle and thread."

Greenpath Encounter
When traveling through the area, she will appear twice, and quickly dash away. When approached at the end of the zone, she tells the player to keep their distance, and comments that she can sense something awakening within Hallownest, and that she knows the nature of the player character, and because of that, cannot allow what the player is going to do to take place, which starts her boss fight.

Hornet has 4 base attacks: 2 charge attacks, one horizontal and done from the ground, and one diagonal done from the air, a thread AOE attack, and a ranged attack where she hurls her needle at the player. The range for her throwing her needle is just below the whole width of the battlefield, possibly giving the player an opportunity to heal. In addition, her thread AOE attack can be a good opportunity to heal once you're safely out of its range. She frequently leaps into the air, where she will either adjust her position, perform a charge, or use her thread attack. Once defeated, she will use her thread to get away.

Upon defeat in Greenpath, she drops the Mothwing Cloak, which allows the player to dash sideways.

City of Tears Encounter
Hornet will appear briefly outside the Fungal Wastes entrance to the City of Tears. Once the player reaches the statue of The Hollow Knight, she will appear again and tell the player to seek "the grave in ash" if they wish to seek a role in Hallownest's perpetuation.

Kingdom's Edge Encounter
In order to fight her a second time, the player must go to a far east area of Kingdom's Edge which requires the Monarch Wings to reach.

"So you'd pursue the deeper truth? It isn't one the weak could bear. Prove yourself ready to face it. I'll not hold back. My needle is lethal and I'd feel no sadness in a weakling's demise. Show me you can accept this Kingdom's past and claim responsibility for its future."

Her attack patterns are the same as in Greenpath, but with 2 added attacks. The first is a counter. When struck when using it, a sound effect of metal striking will play and she will hold her needle defensively. If struck again while she is defending, she will then unleash a wide strike with her needle. For her second new attack, she will use her thread to suspend spiked balls in midair. These will damage the player if touched, and will remain until the player attacks them with their nail.

She has 3 pieces of additional dialogue if struck with the Dream Nail during this fight: "...So strong...You could do it, if you had the will. But could you raise your nail, once knowing its tragic conception? And knowing yourself?... Then do it, Ghost of Hallownest! Head onward. Burn that mark upon your shell and claim yourself as King."
 * From where does it draw this strength?
 * Can it succeed?
 * Is it strong enough?

After winning, the player can enter the Cast-Off Shell and mark themselves with the King's Brand, allowing entrance into The Abyss. However, once the player has interacted with the King's Brand, the shell collapses, and Hornet saves them.

Abyss Encounter
After winning against her in Kingdom's Edge, going to the Ancient Basin, and obtaining the shadow cloak upgrade, she will appear at the top of the area, and tell the player that there are two choices ahead of them: to prolong the world's stasis, or face the source of the plague.

Deepnest Encounter
If the player wins against her in Kingdom's Edge before facing Herrah the Beast, she will appear after the player has undone Herrah's seal. Unlike other instances, she does not offer advice or clues, but is simply there to mourn the passing of her mother.

Final Boss (Spoilers ahead)
In the final battle, if you have the Void Heart equipped, she restrains the boss, which allows the hero to strike the final boss with the Dream Nail, allowing them to fight the Radiance.

If you do not have Void Heart, the basic ending will be unlocked.

If you have the Void Heart and you hurt The Hollow Knight enough, she falls to the floor and the battle continues. After that, she is sealed with you in The Black Egg.

If you beat the Radiance, she will live, but she sees the hero's mask broken on the floor, suggesting that you died.

Trivia

 * Hornet is set to be a playable character in a later update.