In both an effort to clarify (for myself) and perhaps help others...

Here is what I've discerned so far....

Red is either a robot or cyborg who works for The Agency (very tech driven). Blue is an organic life form, a shape shifter. She works for the Garden (organic driven... plants seeds and tends the garden of time). The Agency and the Garden are opposing factions in the time war.

Red and Blue are agents who work in service of their opposing factions. They travel through time using threads (upthread is the past and downthread is the future). And they travel to infinite alternate universes using strands (which both factions share a common numbering system for). They meet different Socrateses and visit different Londons through the strands.

So far the book is 90% correspondence between Red and Blue and 10% glimpses of them on their missions (which mostly consist of manipulating events in time/destroyering worlds as a means to gain advantage for their faction in the war).

Both Red and Blue are part of a collective. Red can hear the others from the Agency. The Garden is patient and plays the long game. They plants seeds here and there and nurture those seeds for their purposes. It can take centuries. They grow and tend and toil until they are harvested by the future and go back to the Garden like fertilizer. They are rooted and embedded into each other like roots of trees in the forest that can communicate and support each other.

Red leaves the collective at age 13 to be alone.... to feel the loneliness. Agents in the Agency don't need to eat or sleep or feel, but Red wants to. She wants more than to be drown out in the voices of the collective. She wants her own voice. She wants words. She wants to eat and dream and experience.

Blue was infected by something in her youth, and her roots were cut from the collective Garden so that the collective could not be infected. She learned what it was to be less than the collective and still be whole. After some time, she was embedded back in the Garden. The experience of being whole without the collective has left her more like a tumbleweed or Spanish Moss, a rootless, organic traveler. Since her infection, she is the embodiment of hunger, not just physical hunger, the insatiable need to feed the whole that is just her.

The letters begin as short taunts and brags to the other, whom both view as a worthy adversary. The letters get longer and more personal. They begin to recognize themselves in each other's words. Their words are pieces of themselves (experiences, curiosities, readings) that they have kept from their faction's collective and have given to each other.

Blue even goes directly against her faction on an upthread mission where she is badly injured saving a 13 yr old Red as she explores her individuality for the first time.

Blue is married to a man (Garden male).

Not entirely sure what the deal is with the seekers. They follow, watch, and collect evidence.

Do my interpretations align with yours? If not, what do I have wrong? What have I left out?