Phantom and Broken Vessel
Was thinking about the parallels between Phantom and Broken Vessel and wanted to write it down somewhere. Note that I am talking more about their narrative function so I will be speaking of Phantom and Broken Vessels as tools used to develop other characters mentioned here. I am not saying they're less of a person than the other characters, just that narratively they occupy less space and are used to tell the story in these ways.
The obvious connection between the two is that they're both the broken discarded sibling of (one of) the final boss(es). Before we're even told what THK and Lace are going through, we see their siblings suffering from the same affliction at a more advanced state.
Their buildup is also quite similar:
You enter a hidden area off the beaten path (Ancient Basin / Mist) set to distinctive foreboding music (wind ambience / eerie organ). This area is deserted other than creatures quite unlike anything else you've seen. It is clear that something is influencing this whole area.
The lightseeds lead you to Broken Vessel and the silkflies lead you to Phantom, which is when the beat finally drops: A staccato rhythm with a variant of the Hollow Knight leitmotif that's adapted to sound more "trapped" or "broken".
I will elaborate on their fight moves down below. Suffice to say the fights relate them to both the Knight / Hornet and THK / Lace.
Once you beat them, their suffering is over. They seem to welcome it (Phantom's Needolin dialogue and journal entry, Lost Kin's bow), but it is not a peaceful death: they clearly felt immense pain as they died (BV reaching out to the Knight, Phantom's scream).
The music is gone; the area is silent. The world is forever changed (Infected Crossroads / Mist disappearing). (For Ancient Basin this actually happens when you pick up Monarch Wings, but close enough).
Another part of their parallel, and something I want to expound upon, is how they act as a bridge between the Knight / Hornet and THK / Lace to help the protagonist (or player) empathize with THK / Lace:
We see the Hollow Knight statue in City of Tears, but THK looks so different from the Knight that it is quite unclear if they are related in any way at all. But Broken Vessel is very obviously similar to the Knight in terms of appearance and half its fight moves (dive, upslash, and dash slash—even its stagger looks like the Knight's damage animation), and this relationship is confirmed with "Lost Kin": You are confronting a copy of yourself that has fallen to the infection that pervades this world.
On the other hand, the longer horns of BV, the infection (and cracked shell), and the other half of BV's fight moves (slam, infection spray, and dash slash again) all connect it to THK. Of course all three also have the same leitmotif. These connections force the player to reckon with the connection between the Knight and THK even without the TE stuff confirming their relationship, like White Lady / Hornet dialogue and Birthplace scene.
Note that Broken Vessel is supposed to be required for all endings, though it is skippable in ending 1 due to the Watcher Knights statue pogo (also technically skippable in ending 2/3 but nobody other than Skurry is insane enough to do it). So in ending 1 with no King's brand (therefore no Abyss, no White Lady, and no Voidheart), it would've been the biggest hint about the Knight and THK's siblinghood.
With Silksong, of course the game tells you to think about the similarities between Lace and Hornet way earlier with the Lace 1 bossfight and music, but even then, Phantom's presence adds some weight to the idea. As you fight Phantom, you notice that Phantom's fight moves, other than the slam, are super similar to Hornet's in the 1st game (they launch the pin and go after it, unlike Hornet who either launches or goes with her needle, but mechanically it works the same way), and they are one of the most vocal bosses who clearly taunts with each move, just like Hornet in Hollow Knight.
On the other hand, Phantom is of course clearly connected to Lace through their designs, their fencing moves, and their voice lines. The Needolin scene confirms their relationship. The moment you realize that resemblance with Phantom, there's an additional channel to connect Hornet with Lace: They are also similar in these ways, just a little less overtly so! Lace's outward childishness masks some of that similarity, but Phantom sounding clearly older and very similar to Hornet puts that connection up front. And that encourages the player to be on the lookout for any other similarities Hornet and Lace may share, say, having the same (ancestral) creator, or having guarded the corpse of a kingdom.
In that sense TLJ is the "saving the world" path and Phantom is the more personal "understand Lace better" path. Lace even shows up sooner in the Citadel if you do the Phantom path—you don't spend time taking in the silent Citadel at all, you just immediately run into her. With that in mind I think it is a pity that Phantom is entirely optional for TE (while TLJ is required), though that does mean Phantom can survive the events of TE and hopefully have a chance at life.